This is very important please help?

My mom and dad are divorced. My dad has ruined my moms life well tried many times pretty much. mostly finacially. He left me and my 2 sis while my mom struggled and now that my mom and rest of family have a better life he trys to come back in it by really bribing us with money and materials. of course being teens we accept them but we are only using our dad. My mom doesnt like it saying that we love him more than her. Cause being an only parent she had to go through alot of trouble with us especially my 2 sis. But anyhow we telll her that we love her but she doesnt believe that at all.With our mom we had little money to really enjoy our childhood to the prime so my siblings though it would be okay just to use him. Now my sis is graduating but she wanted my dad to go to her graduation. Mom got pissed and said that she wasnt going to her graduation thats about to come up if the man is there. Saying how he is going to front in front of everyone else when he did do shix to raze us. That he was taking all the glory like he was this so called great dad. Now our mom pretty much hates us. My sis invited the man to the graduation because for once in her life she wanted the whole family together. I told my mom that she should respect her decision but she still doesnt care. She refuses to go out to eat. Im tired of their stupid feud and i am bout to go mad. For one day i just need the to stfu and be neutral for once. Cause if its the last thing i do i will make sure that my sis has the best graduation. I know after all that my sis shouldnt want to talk to the man after all the times he lied to us etc. but she told me thats she pitys him. Please i need feedback