If she keeps in touch regularly, there is less chance that it's an abusive relationship. In most such relationships, isolation from family and friends is part of the pattern.

Exactly what did your brother report?

Do you know any of your sister's friends in Florida?

Do you think that anyone in this triangle (sister/boyfriend/brother) is mentally impaired?

If you google "help for domestic abuse" many websites pop up as well as the national domestic abuse hotline. You can call and see what they suggest, no harm done, and you can take or leave their advice as you see fit. But if something does escalate, you may feel more comfortable having some resources in hand. I couldn't live without my sister. Good luck.

You are right to be concerned but your brother needs to give specific details for you to step in.

In the mean time, keep communicating on a regular basis with your sister.

I volunteered for a domestic abuse organization for some time years ago. Anglophelia's comment is very insightful. Beyond that, there's little that any outsider can do unless your sister is willing to come forward. You can assure her that she'll always have a place in your home if the need ever arises. You can encourage her to keep a separate bank account that only she knows about. You can encourage her to have a travel plan including clothes, etc stashed away if the need ever arises. But beyond that...unless she's ready to share her situation [if there is one] with you...that's about all you can do. Outside involvement from relatives or friends can often escalate a dangerous situation when that's the exact opposite of what you intend. It's a walking on eggshells situation and the best that you can do is stay in touch with her on a very regular basis...and listen for signs that she's not completely content with her present situation.

I Agree with Susie. It's not fair to your brother to be mysterious about what's going on.

I agree....I wouldn't do anything unless your brother gives details. If they had a little argument, your brother, not married, may not be the best judge on relationships.

I tried to get him to give me more details on the phone call but he refused. He also made me promise not to tell anyone, including her, what he said. It could just have been lots of arugments. He tends to tell tall tales. My sister's man friend is 70+, has a lot of health problems and money problems. He would be out on the street if not for my sister. There is a lot of tension and drama there. That is why I don't visit. But I do worry, she has been in abusive relationships in her youth. I would think it would have shown up in 25 years if there was something going on.

It could be verbal abuse but not physical abuse. My response to your brother would be, "I'm not doing anything until you tell me everything." Put the onus back on him.

Brother sounds like a pot-stirrer. I agree with gray wings. Tell him you need more information before you see any need to take action. Better yet, what's wrong with HIM taking action?

If you do decide to question your sister, be aware that if you call her at her home, it is very likely her bf will be there and that she cannot talk freely.

Wow, how your question sure has triggered a flood of comments from amateur buttinskys and amateur sleuths.

Until you've heard something specific FIRSTHAND and have been asked about it (from and by your sister), there's nothing going on that you are entitled to jump into. Keep in touch with her as you have before and if she has something she chooses to talk to you about, fine. If not, you have nothing more than innuendo to deal with and that leads nowhere.

If you're really concerned, go visit her and you will either see or not see what your brother has described with uncertainty.

"But I do worry, she has been in abusive relationships in her youth. I
would think it would have shown up in 25 years if there was something
going on."

It is fairly common (or maybe not uncommon is a better way to put it) for men, as they age, to become more abusive. Loss of their health/work/money/power/sex life, sometimes brain changes caused by dementia or other health problems, can cause changes in behavior toward others, especially their spouse.

Citing his health problems may be a way to open a dialog with her about it...something like, "One of my friend's husband had (name his health problem, copd, arthritis, or whatever....) and she started having more problems with him becoming difficult to live with." Or something similar...but more subtle, just to maybe give her an opening, and perhaps some insight into changes in his behavior, if there are any.

Just remain open to listening to her, don't judge him (or she may feel forced to defend him) and keep your communication with her open.

"It is fairly common (or maybe not uncommon is a better way to put it)
for men, as they age, to become more abusive. Loss of their
health/work/money/power/sex life, sometimes brain changes caused by
dementia or other health problems, can cause changes in behavior toward
others, especially their spouse."

I'm not an expert but a quick internet look around suggests that the rates of dementia and Alzheimer's, and the effect of same on the behavior of such individuals, doesn't show strong strong gender tendency differences. So it's as likely to go one way as another. Abusive behavior is also present in many females. There's maybe a bit of misandry in the quoted comment, which is unfortunate.

Elmer is spot on. I saw that at a women's shelter sponsored meeting group. Older women would be brought in, sometimes by well meaning feminist daughters, and it often turned out to be dementia or Alzheimer's. I remember one woman in particular. She and her husband were in their late 70s and mid 80s. He had become more sexually aggressive.
It was obvious to me they adored each other. He actually drove her to the meeting. I got to know them both. The facilitators would encourage this sweet 78 year old woman who loved her man to pieces to get a divorce since he was being "abusive". I pushed and pushed and finally they got them some senior advocates to step in. There is an awful lot of men bad/women good dogma in the women's shelter industry.

I'm sorry for your worries -- evidently not new regarding this sister? It's unlikely that she will suddenly discover higher self-esteem. Tell her you love her and care about her. Be a non-judgmental 'ear'. (Be neutral about her live-in.) DO you know any of the people in her life, beyond this man?

Several year ago we donated a full set of new-ish pots and pans that wouldn't work on our new induction cooktop and several other things to a local women's shelter that was just about to open. An acquaintance hosted a party to thank those who donated to the cause, and at the point when the director of the shelter came in to talk about the shelter and its plan they asked me to leave the room to prevent me from learning the location of the shelter. I complied, but I a little bit amused and a little bit offended. I'm not sure if their reasoning was "I'm a man, so of course I'm a potential abuser" or something like "well he's OK, but he might blurt out the location when he's out drinking with all of his abuser buddies." It was a "Wow! Really?" moment for me.

My son has been on a jag over how men who are abused have nowhere to go. He'd could have a huge bull session with you Bob! He's POed about it. Worse, how men being abused by women can be dismissed on hotlines and told to man up in different settings. It's not right. Abuse is equal opportunity to victims and aid. It sucks you were asked to leave.

Bob, that's ridiculous, most especially because you were there as a supporter.

I've encountered many DEVOUT feminists who are unapologetic man-haters, there's no doubt. Including one of my wife's friends, from whom I take no BS when we're together (as you can imagine). I call her on every outlandish comment she makes out of habit and it usually registers with her. Our being together is good for her and good for me too.

I think the term feminazi is unfortunate but the meaning it conveys is clear.

My sister's man friend is 70+, has a lot of health problems and money problems. He would be out on the street if not for my sister. There is a lot of tension and drama there. That is why I don't visit. But I do worry, she has been in abusive relationships in her youth

The brother didn't have to say anything first, this shows that you were already aware of a problematic situation being present. Why wonder if the brother is exaggerating? He doesn't have to.

she has been in abusive relationships in her youth = established pattern probably requiring professional help to break. As you are not her parent or guardian you can't do anything about it unless she asks you for assistance.

At one of our local resource places they hold support meetings for women. The same place is used for the male abusers who are mandated by the courts to take anti-violence classes. So the abused women pass the abusers on their way in and out.

The women's facilitator also teaches the men's groups. She say the "classes don't work because you can't change men like that". I asked why have them then. She said "so men will learn not to abuse women".

Crazy.