I share custody of my DS with my ex. Years ago, my ex husband remarried and they had a kid together.
I'm really happy for him and his wife and while it felt weird for a little while that my DS has a half sister I had no say in, it's also been great for him to be a big brother.
There's just one thing. When my ex got remarried, I'd always hoped that we could be adult enough that we could all get on. I get on with my ex, that's 100% fine but his wife has always treated me with suspicion. If I'm dropping off my son or something he's forgotten etc, I'm never allowed beyond the door, even if it's chucking it down with rain and normally I know my ex tries to manage it so I don't turn up when only his wife is in (so it's rarely her who answers the door).
I'm not forcing anything and never would but at some point in the future, our son might marry and I don't want it to be weird when that day comes. Also when we first split up, we'd occasionally meet for a brew or I'd stop for a cup of tea when dropping him off so we could update each other on what he's doing, any issues etc and genuinely coparent.
In contrast, my ex has met my partner loads of times. Even being at the same birthday party once which was fine. They got on fine and I know it would be no issue at all in the future. My partner isn't around much when my ex comes over but is completely unbothered when he does. Doesn't mind if he walks in etc and wouldn't mind if he stopped for a few mins.
Am I expecting too much? I don't see my ex's wife as the evil Step Mother. (What a horrible trope that is.) But even when I picked up our DS from their wedding (at my ex's request because he wanted me to look after him for some additional days for their honeymoon) I was told by my ex that I wasn't allowed to come to the venue but to wait down the road for his sister to bring him to me. (I get why she wouldn't want to see me on her big day but I could have waited outside the venue and wouldn't have been any bother.)
I'm very careful how I speak about my ex and his wife to my son. I always back them up. There's not anything bad or concerning that comes up but I don't get the impression he has the closest relationship with her. Not that it would really matter anyway. He doesn't need to have four parents and I understand taking on a level of parental responsibility for my DS is not something that comes easily to others (and I don't expect it).
I suppose it's not a big deal. But I don't want it to become a big deal either. How could I reassure her I'm actually not horrible and in no way a threat?