Hi, Michele. What an interesting question. I am religious, but when I put myself in your shoes; I can see the predicament you're in. I think it's very sweet of you to entertain going, even though you don't want to. (Your husband owes you big time for this!)
At our church, Baptist denomination, we only do communion about once every 2 months or so. Actually, we're told NOT to participate if we don't want to because that would not be the right spirit for taking communion. There've been times I haven't taken it, when I knew it wasn't right for me.
There are several times throughout our service, when we stand; for praying or singing. There are often many people who never stand and some who never sing. No one ever thinks anything bad about it. I always figure that people have their reasons, and it's no big deal at all. My husband has very bad back problems, and he never stands. Sometimes, I sit too, just so he won't feel alone.
I've only been to a Catholic service once, and that was when my sister got married. All I really remember is the constant kneeling, up and down. I did join in it though, out of respect for my sister, even though I didn't understand it.
So, I would say that you should stand when they stand but don't take communion. Unlike standing, Communion would be saying that you believe what they believe, and that wouldn't be right.
I feel for you having to go somewhere you're not comfortable. I've done that many times myself, in different situations, and I hate it when I have to do it.
Sometimes, it's just easier and for the best to keep the peace, though, isn't it?
Hope this helps a little though,
p.s. I really admire you for going. I hope your in-laws can appreciate you for it.