Help me save my family graduation party?

First let me say I'm sorry that your sisters have sabotaged your events so often. And congrats on graduating!!!! Since you "could care less" about celebrating with them and you're doing it only for your mom, why not just take your mom out to dinner to say thanks to her alone. If that's not an option then go to your party with a new sense of humor. When the sisters get to griping and complaining, don't let them get to you. Smile ,even when you want to yell, and tell your sis that you love her, or how pretty she looks, or something else to "throw her off". Don't take them seriously and you may diffuse the situation