My late husband left his first wife after she spent the night with a colleague. Two years later, she tried to get him back. (I don't know whether her BF ever knew about that.)
After we got married, she took every opportunity to stick her nose in. I put up with it so as not to interfere with my husband's relationship with his adult children. (They were adults when the pair separated.)
The one boundary that I set was that she was not to set foot in my house.
Looking back, I wish I'd set firmer boundaries. I spent my marriage of 27 yrs 'being nice'.
I'll not hog space here by going into it all, but I was manipulated into letting her represent the kids at my husband's funeral. 3 days later, when I answered the door in a daze of Diazipam and Zoplicone, she was standing there making bizarre small talk...until she asked what I was doing with my husband's ashes.
She was trying to manipulate me into burying them in a cemetery within easy reach of her latest partner's home...and 3 hours away from my home.
Based on my experience, OP, your husband is probably oblivious to his ex's machinations. (My husband realised that his ex was very possessive of their kids, but I don't think he realised that she was still digging her claws into him.)
After my husband died, my husband's DIL told me (over the phone) that his ex was 'devastated'. She was taken to and from the funeral by her 4th partner.
Had I known what I was going to be put through, I'd have put my foot down years ago.
I'm not saying that your husband's ex is as bad, OP, but I reckon you might have every right to be wary of her. (Apologies - I may be projecting.)