I know I’m being U but need to vent and need you guys to help me snap out of it
DH was previously married to his ex wife, they were together 5 years and married 2. DH ended the marriage as his ex became more and more controlling (eg she kept hold of all the money, controlled all the finances and he had to ASK her for money if he wanted to buy something) This all started when they decided they were gonna buy a house together, and do it up. His ex wife said she’d take control of the savings and do the budgets for renovations etc and this eventually turned into DH giving her his pay check then having to ask for money back if he wanted to buy a t shirt for example. DH got sick of it and they divorced.
Fast forward to now and SIL (DHs sister) is still really close friends with DHs ex wife so we occasionally have to see her at gatherings such as bbqs, or our nieces christening. His ex wife does not like me and makes a point of ignoring me at functions but will be ALL OVER DH’s family like a rash. At DN’s christening his ex wife sidled up to my BIL and said “it’s great to see you, have you missed me at family functions?” within ear shot of me and DH. She has also said to MIL “you’ll always be my mummy in law” (gag, she’s 32 years old and talks like a child) which made MIL feel uncomfortable as it was in front of everyone.
At a bbq at my SILs last summer, my niece came up to me and said “Aunty Dwight will you come play with us” and DHs ex wife looked up and said “I’m your aunty too! What about me?” 
She has also given my SIL a birthday card last year that said “to my SIL” on the front and she had written inside “from your favourite SIL ever!” 
Tomorrow my SIL is having a huge bbq / party for her birthday and of course DHs ex wife is going to be there. I’m dreading it. Every time she makes a comment about how she used to be a part of the family I want to slap her. DH gets annoyed too but says we just need to ignore her and I shouldn’t let it bother me but I feel like she tries to push me out of my own family.