I am at a loss... I just don't know what to do any more.

Some quick back ground.

I divorced with one three year old and married again when she was five. Incidentally the first thing I did on our first date was show my (current) wife a picture of my daughter.

The years have now passed and my daughter is 12 and as time went on things got worse. My wife and I now have three other children together at 4, 2 and 5 months.

The only arguments we have are over my daughter, her step daughter and they are usually blow outs. They just can't get along. My ex is a decent mom but seems to lack any real emotional attachment and I have my daughter 1/2 the week, every week encompassing the weekends. When it is all said and done, I am the anchor in my daughters life.

Clearly SM and SD resent each other but if I were to rate it fairly, the SM is about 75% of the problem and SD is 25%. SD tends not to give SM any respect and in turn, SM doesn't ever cut her an inch of slack on ANYTHING and even when I don't agree I am expected to back SM on everything. SD isn't really disrespectful; she is a 12 year old girl being a 12 year old girl. But in SM's eyes everything SD does is purposefully to undermine her and to make her mad. SM really takes it to heart... like it is all meant to piss her off and there is some underlying plan against her

SD is truly a good kid but even the most minor infraction is greeted with a wrath that I don't quite understand. I don't think that SM even realizes how negative she is when speaking to SD. For a long time I thought it was just me but over the years SMs mother and family along with many friends have noticed how hard and flat out mean SM is to SD. Then SM will do one nice thing for SD and expect some sort of award but SD just doesn't really care anymore. Those are very odd occasions anyway. SM will go out of her way not to have SD there if I am not there too... like, she would not go get SD today from her Mom's house. SD is there all day by herself and SM is home, and SD comes to my house today anyway. She would be a few miles from there where I would have to drive an extra 45 min after a 40 min commute from work to do it. I even tried to bribe her by saying I would pay for them to go get manicures... she said that if she did, I would make her take SD so no. To make matters worse SM's Mom offered to get the kids from daycare today and take them to her house this afternoon and asked if she could go get SD from SDs moms house and SM told her no, she could just ride public trans to our house later.

I think that SD is so sick of getting jumped on about every little thing that she just doesn't say anything to SM anymore for fear of the outcome. I don't think that SM even realizes how she treats SD anymore. SM can go from sweet and nice to everyone else to super mean to SD, then right back to super nice to everyone else. I just don't get it.

Any discussion that SM has with me or anyone else that relates to SD is almost always negative. So much that I have to leave the room last night because I could not stand listening to her bash SD anymore. If I say ANYTHING about it I am accused of telling SM how bad of a person she is.

I am lost...

This is going to end our marriage.