Both of my brothers are married and I am inviting both of my SILs parents to our wedding, since I know them and we have many family get togethers with them.

Also, my parents go to church with one of my SILs parents, so they are friends with them through church activities.

I am inviting one of my SILs siblings and family to the wedding, because I am friendly with them and have seen them at gatherings, as well.

My other SILs siblings I am not inviting, because I barely know them, since my brother has only been married to her for about 2 months now.

I think it depends on how well you know the people, really.