Mother in law wont attend daughters 1st birthday because her ex husband will be there.

There is so much context to this. Im going to try and make it short. My mother inlaw kickedher husband out of the house and "broke up" with him when my husbandand i were in 10th grade. Her ex husband lived across town and always went to her house to help with things and the children which they have 3 and at the time the youngest was 16. After 4 years of living separatelyshe divorced him. He started a new relationship after his ex wife refused to take him back 5 years later. He then moved across country to be with his now wife. His 2 kids and my husband are all adults now. My husband is the only one to keep contact with his dad (her ex husband) because his mom has told the other 2 children (grown adults) if he loved you guys je would have stayed ect. Crazy things she has said to them. My daughters 1st birthday is coming up. This is a big deal for me because the 1st holidays were lost to house hoping and pleasing everyone else. My mother in law is refusing to come to her first birthday because her.ex husband and his wife will be there. Mind you she has moved on too and is in a 7 year relationship. Her argument is we (my husband and i) are choosing her ex husband over her kids (my husbands.very grown brother and sister). My mother in law is expecting us to have a party the same day as the one im having for my daughter. She gets very ramped up about this. Everything we say she has something to shoot right back at us. Ive explained to her there will be no drama at the party and my parents who had a very nasty break up will be attending despite their differences they will be there to celebrate their granddaughters first birthday. I also feel like if she gets her own party for my daughter its "unfair" to my parents whom are showing up despite differences. I just want to have a birthday party and i dont know how to handle this situation without causing family tension. Sorry this is long its just weighing heavy on me. Fan