I'm not going to my sister in law's wedding
Why won't you go to your sister in law's wedding?
I just don't want to be there, I hate all that shit.
I'm not a big fan of weddings either, so I understand where you're coming from.  However, your family is going to be mad because people usually expect other family members to come to weddings.
Family is only as good as it is to you, so why does it matter?
I just think that it's nice to go to family weddings even when we don't want to. I'm sure your sister in law would really love to have you there.