I'm not going to my sister's wedding
Why don't you want to attend?
I hate my family and their gatherings.
It's okay to want to stay away from large gatherings, but this is an incredibly important event for your sister, and she will want you to be there. You should be willing to support your family on their special occasions, just as they would do for you.
I'm not sacrificing my sleep and will be cancelling my plans to attend a shitty wedding.
It is selfish for you to do that and be so inconsiderate of you t your family. You should go there and them reward yourself after to set yourself an incentive to go.