I understand what nico is saying. But what about if you have drama llamas in your family/group of friends who would throw a tantrum if they found out they weren't invited?
Example: One of DH's sisters lives in Virginia. Obviously, she and her family will not be attending Son's and DD's birthday parties. But I know for a fact that if she doesn't get an invite, she would call MIL and be all OMGWHYDOESNTTAWLIKEMEEEE?!.! Yes, this is a grown woman, but shouldn't be too surprising, given what you already know about DH's family.
I just always send her an invite and bypass the drama completely.
Do you do the same?