Shrug and listen if you want to listen, or do what you want with your time during their visits.

Usually, if people are doing such things intentionally they would have enough normal variation in their routines that you could tell. That is, they would talk about things you could participate in sometimes-- it would be highlighted that this was a strategy or somewhat intentional pattern on their part.

Different people are taught different things when they grow up in their different families. There are spoken and explicit messages, and there are unspoken behavioral examples set by familiar adults. And, sometimes there are just personal 'problems' people can have which keep them from just being naturally able and adept at giving others a feeling of inclusion. (Even among the obviously prosocial and empathetic, some people are probably felt to give 'better' hugs than others).

If you have a choice, it's probably better in the long term to ascribe their behavior to personal differences they seem to have. It may or may not be 'just how they are' but if they are adults, it usually works better to figure that in their own heads, they are at least as sincere and trying their best as you are. The evidence that they don't have your brain and your background and upbringing (and genes and whatever else) is that they behave differently.

one answer to your son's question might be 'because they haven't learned how'-- relationships in practice though are at least two-way so either side can see if there is an interest in building one, and can respect the other's point of view

P.S. The idea is to keep them humanized in your thoughts, words and deeds. If you want to try to engage them in conversation to build your own one on one relationship, try asking them out for coffee or some activity you can all enjoy. Basically, one way to build a relationship would be for them to just include you in their conversations, but it sounds like they have a stable established pattern which kind of works against that. You can give it a try if you go out of your way and change your pattern enough to try to get you and them some enjoyable one-to-one time.

Could it be that your husband and his family aren't all that comfortable with each other? In my experience, people who enjoy each other tend to let conversations wander all over the place. But, if your DH and your inlaws stick to same boring subjects, it just might be that they have nothing else to talk about. And if it's a strain for your inlaws to talk comfortably with their own son, they aren't going to be able to talk much with you either.

Have you talked with your husband about how this makes you feel? If so, what was his response? If not, you should. Maybe he is not aware of how this is affecting you?
I had a similar problem when I was first married. My husband and his siblings are very close and I always felt like an outsider. I talked with my husband about it. And things were much better. He made more of an effort to include me and I also did more things with my brothers and sisters in- law so I could get to know them better. It helped a lot.

I have been thoughtfully considering your responses and continuing to pay closer attention. What I have noticed is that they don't just do this to me! It seems to be one of those "wherever two or more are gathered" things. The three of them own a small company where we all work. There is a lunch room with a large table at the office. I and two other employees often eat lunch there and the "owners" often do too. If there is only one "owner" and one or more "employees", the converstation is inclusive. If another "owner" joins the group, the conversation immediately shifts to "owners only" (non-business) topics like their family in their homestate that none of us know, common history and experiences, family history, etc and they seem to expect the "employees" to sit and listen.

I think it is more of a dominance thing on their part (they all have a very 1950's orientation). It is unfortunate that it carries over outside of the office. They are way too comfortable with each other. I have done plenty of things with my MIL one-on-one, there is never a lag in the conversation and she seems to really enjoy it. There can be no talking to my husband about this because he will take it as criticism no matter how delicately I put it and become angry and defensive.

Really, I think I am just "O-D-ing" on my in-laws. Being neighbors, family and working with them too is getting to me. I do wonder about the part of the vows that say "leave your father and mother...forsaking all others and becoming one". I guess I was looking forward to it and did not expect "others" to figure so prominently in our daily life. My husband, by the way, is happy as a clam with the way things are and keeps telling me how wonderful everything is.

My solution is to shrug it off and look for a job somewhere else. I am hoping I will feel differently if I have less contact them and have the opportunity to be with other people. I guess I am feeling very smothered by his family and hurt that he places them "above" me...shows that "blood is thicker than water" I suppose.

Frankly, I'd get up and leave whenever the conversation went that way. If by chance they notice that you don't hang around and ask what you're doing, just tell them that if they'd like to include you in the conversation you'd be happy to visit with them, otherwise you have things to do! Sounds to me like they just don't know any better. Although it may look intentional I think it's just bad manners. If they see each other at work all day, they're just used to discussing things among themselves.

You too? I thought it was just me! My MIL and FIL do the same. After 11 years of marriage I have heard all their family stories told so often I could recite them by heart. If they are going to exclude me from the conversation I wish they would at least get some new material. In my family when inlaws are present we discuss matters that include them. We even ask them questions (!) and try to get them invlolved in the conversation. I was brought up not to do so was rude. In my husband's family they themselves are the topic of all conversations. Usually events that took place 20 or more years ago are rehashed over and over. Whew! Sure feels good to vent after 11 years. Thanks for bringing this up. :^)

To me that doesn't see abnormal for a family, sounds like they are discussing family history. When I first married my husband we were both in our late 40's, they made an effort to include me in the conversations, but when we were all comfortable with each other, they discussed mutal friends, distant relatives and events that I didn't know about. I just sat, listened and learned. When I did meet some of the people they were discussing I felt I knew them.