Toosassy you are completely right in your post.
My dp's ex wife is like that. I was the first woman my dp has gone out with since their split 5 years ago (he was single for 3 years). She's never acknowledged me at all (spoke to/met/seen) which is fine of course, she doesn't have to and it's not something I would queue up for either. But her behaviour towards my dp has spoken volumes to me since she found out he had a girlfriend (we've been together 2 years now) where she has mainly used their son as a weapon and basically tried to tarnish him as a person, I assume to put me off. It appears to be a case of she doesn't want him but she clearly doesn't like him having someone else either.
When I first met his son she had a go at my dp for not telling her he had a girlfriend (I don't know the correct protocol for this because I assumed it was none of the ex partners business, especially in the circumstances where he hasn't just rushed from one unsuitable relationship to the next, he has remained single to properly move on in his head etc.)
Then his son, who was 6 at the time, said things like "you can't tell me what to do," which had clearly come from his mum/her family. My dp explained that as a grown up, I can, within reason tell him what to do.
It got worse when their divorce petition finally got going. They had started proceedings 2 years prior but it never went further than the initial petition. My dp explained that they had mutually agreed they should get divorced as they had, at that time, been split 2 years, but one thing or another happened and it just never got done. The original petition had nothing other than they had been split and living apart for 2 years (I saw it.) Then another 2 years on and he had been going out with me for a year, the divorce got put back on track. This time she sent another petition and wrote a great long assay on it basically accusing my dp of all sorts of abuse, emotionally and financially. My dp was so annoyed and it started a whole debate with him obviously refuting her claims against his name.
We went through a whole load of shit with that and I saw a number of bits of proof with showed her to be a liar (she did all their accounts for instance and was in charge of all their money.)
My dp sent a text with his annoyance (it was a normal reaction text, didn't have any threats or abuse in it, just "why have you said those things about me when you didn't say them before.." kind of thing) She used that to withhold him seeing their dc for 3 months until they could go to court to gain an access court order. That was bloody tough. She still tried to tarnish him to caffcass who themselves said any accusations of abuse are historical and very minor at that.
The whole thing was a complete farce from start to finish. She went from happily communicating and seeing my dp at pick up/drop offs to (as soon as he got a girlfriend) being difficult, making ludicrous accusations, trying to paint my dp as some sort of abusive bastard to the point that she requested separate entrances and waiting rooms at the court for the child access court hearing. My dp said on the rare occasion he is allowed to see a school play, when she sees him in the school hall she acts all frightened of him to put on a show for her friends who she has clearly made my dp out to be a right abusive bastard which has only come about since he has a girlfriend.
It can't even be said that I've stirred the pot because I've never so much as seen, spoken to or got involved with her. I've supported my dp but at the same time kept out of it. Part of my wonders if that annoys her more? I'm definitely not out to be having spats with someone's ex, but all this effort to tarnish his name has probably made her wonder why I haven't either dumped him or said something in defence to her?
The status quo now is someone else has to pick up his dc from her and she is keeping up this facade of being frightened by him. She wasn't that frightened of him when they were still sleeping together up to a year after their split.
Step mums can be awful I agree but so can first wives.