How to deal with boyfriend's sister drama?

I need some serious help, I have been with my boyfriend for over three years. We love eachother, we have a beautiful daughter together, and I USE to get along with his family. I found out his sister is talking smack about me and even our daughter (who is HER NIECE!). Now she and their mom has planned a party for my boyfriend and didn’t include me at all or our daughter, they told him that there will be a party for him Sunday, but I am planning my sister’s 40th birthday party for that day. I cannot change the date for I sent out invitation over 2 months ago and they knew that we will be celebrating his birthday the Saturday before my sister’s party, I invited them but they told me they were to busy and wouldn’t be able to do it. They never celebrated his birthday (it has always been me and my family) before now it seems since my daughter was born they just flat out ignored us and my boyfriend is in the middle. I don’t know what to do, I want to punch his sister’s lights out (one thing to talk about me but our daughter is another thing), I also want to point out we found out from his father (not his sister’s real dad) that she is saying things about our daughter, I confronted her and she made my boyfriend’s and her other sister’s life a living hell, my boyfriend is closer to the other sister and threaten to kick her out if I talked to her the way I did (I asked her the question and she snapped and I lost it). She tries to play the victim telling everyone that I threaten her and it all has to do with her bad childhood (she was a spoiled brat, the whole family knows she had a good childhood). I need help fast or else I will have no choice but cut all ties with my boyfriend’s family and my daughter to never meet them beside his dad.