Is inviting both twins necessary?

My son has girl twins in his grade. He is friends with A, but not friends with B. He was in the same class as B last year and they were constantly separated for bickering. We have had A&B over to play with my son and stepson, and B always runs home unhappy about something. My stepson and B get along very well, although other kids coming do not get along with B. I'm having a small movie night for my son's birthday and he left A off his invite list. When I asked why, he said he didn't want to have to invite B. Which made me scratch my head.I don't have twins so I want to be sensitive to them but my instinct tells me we can certainly invite one and not the other, but then my guilt tells me that's not very nice and will make B feel excluded. What is the right thing to do? Do we invite both since stepson is friends with B and it's the nice thing to do, or do we invite neither since my son would rather not have B there, or do we just invite A and follow my belief that twins do not need to come as a package. By the way, this is 2nd grade. via /r/Parenting http://ift.tt/2jW663b