I'm going to skip my sister-in-laws fathers funeral
Why would you skip the funeral?
I have no clue who that is
You should know who the father of your sister in law is. I am sure your family will want you to be there.
Dude, I just don't care.
It is bad not to be there for family when they need you. You might not know who the person is, but sometimes family just needs support. It is up to you whether or not to attend the funeral, but think about your family before you decide.