I don't want to go to my sister's wedding early
Why don't you want to go her wedding early?
I think the reason is quite obvious: I don't like stupid weddings.
I understand that it's sometimes boring to go to events that you don't want to, but she is your sister and this is her special day. As her family, you should support her and help to make this day positive for her.
It's not a question of responsibility. I just don't want to see this whole thing.
It's expected that you will support your family and the events that they have. Doing this will show that you care and it will improve the bond between you and your family members.