This is going to be long, sorry!
A bit of background: DH has form for trying to force things a bit between me and his kids. Mostly when we first got together but it still happens a bit (together 5 years now) It's always with the best intentions but things like telling them to hug me goodbye (I'd prefer to let them decide they want to when they are ready) organising the day so that I end up being there when he drops them back home (stood in the doorway like a lost dog not knowing what to do) and trying to get me to meet the mum to pick the kids up if he is busy. Some of these things are fine (I am now happy to meet the mum to collect the kids, the first time was a bit awkward but we managed)
We went over to their house for one of the kids birthdays and it was awkward and uncomfortable, they were polite but tbh they weren't that arsed, and it seemed like the only person who wanted me to be there was DH.
So as not to drip feed - things with the mum were very difficult in the first couple of years and I stayed out of it as much as possible. Things have settled now and we all get along OK, she will collect the kids from me if DH is out etc. To be clear, I adore the children, they're wonderful, kind, funny kids and lovely to be around, and they and my child get along very well. I enjoy having them over when they come to stay.
Today he's gone over to see them and asked if I wanted to come. When they come to us, I am an extra, they're here to see their dad, not me. I get involved with things and we do family days out and I play with them etc, but AIBU to think I shouldn't be inserting myself into their home life like that? He doesn't even ask the mum if it's ok, just turns up with me there and it's awkward and I feel like I'm intruding. I didn't like my stepmum much and I just wanted her to back off. I don't want that for them, so am I just projecting? Or am I right to 'know my place'?