My ex refused to allow my children to attend Boy Scouts. Why? Because I wanted them to of course - no stated reason just "NO". Amazing what women will do to their children to cause pain or frustration. I swear, some of them would sacrafice a kid on a alter if they could get away with it just to get revenge. And who initiated the divorce or caused it seems irrelevent.

Who has custody? Her? End of subject with exception: If the activity is on his weekend visitation then it can continue. Shared - she can effectively veto it by simply not letting him attend when she's in control.

Family court can order her to stop interfereing but unless Dad can represent himself it could be expensive.

On your care days you can do whatever reasonable sports you want them to do, on her care days she can do her choices.
However she get's no say in either you can turn up or not to any events if it's on your care days.
This means, if DH is training SS on his days, you can go.If anything happens of her days, don't go, but because of BM's attitude I would also not encourage DH to go.Sadly , with BM wanting to exclude you , there must be a much clearer regulation and who has him what days, which can't mean that your SO still can go and do things for SS on other days that are not his care days.This may mean that SS misses out but the one who should feel bad about it is BM.
In our family we always encourage BM to come to SD's school/Sport events even if it is not her care days and vice versa.Silly that this BM sees it as a threat, but on her days she gets to decide I am afraid.

Personally, when I have attended sporting events of SO's I did not make signs, jump out into the spotlight, etc. because it's not necessary. I would probably see it as "too much" as well, if I were the bm. The kid knows you're there, dh knows you're there, that's all that matters. Actually, I don't even make signs for my own kids... Just being there is enough to piss her off. I've been there, done that!

BM threatened us with that too. Because she wasn't "consulted" in signing SS10 up for stuff, she wasn't going to bring him while he was with her. We said "ok, thats just gonna make YOU look bad to ss10". And it did. SS10 knew he was letting his team down, he was letting us down and he was unfortunately the target of questions from his teammates like "why weren't you here for the game saturday?" etc.
I say go for it. Let her hang herself. She's only going to look bad to him. Sign him up in your town. Bring him when you have him and if bm chooses not to - thats on her.

I would personally do what ever the hell I wanted, with no regards to BM feelings. One thing I learned about being a stepmom is never ever back down to BM. Whatever she fires with return it to her doubled, she will eventually tire out, & may actually be afraid of you & will learn to stay the hell out of your way. She's the mother right, and we (SM) should not interfere with them or try to take their place. So the same goes for BM, they should not interfere or try to replace the SM, because we all know that SM are a step above the BM! LOL that is why BM are all so damn crazy, cause they are constantly competing with us SM but we are always a step ahead...

She is jealous, obviously. It sounds like you're the kind of mom I am... you can't help but be involved! The things you normally do (signs, snacks, etc.) are above and beyond for her. Maybe even just showing up is above and beyond to her. Its terrible how many BMs use the kids in these ways. Our BM would never do more than what is expected (if that) and its sad because the kids suffer. Your SS will see you & your DH's efforts, how you're doing what you can to have him in sports and make it a good time while BM is trying to take it away. He might not see it now but he will when he's older. My SS11 is starting to see how his mom lies to us and keeps him and my SS9 away from us. Just do what you're doing. Unfortunately BM will do the same but will hurt your SS in the process.