I don't feel like going my gf's sister's engagement
Do you have a reason to not want to go?
My girlfriend's sister married a guy similar like myself and I don't want my gf to get the idea.
You don't have to like it or want to be there. But you should go to important events with your gf to strengthen your relationship.
I don't care, I am not going and I'm going to tell that to my girlfriend.
If your gf is okay with that then go tell her you won't go but there's always a catch somewhere and it will be 10 fold.