Should I be insulted or worse?

Dating someone for over 2 years. He has been invited to all of my family events, local and otherwise. I also took him on my family vacation last year to an Alaskan cruise. Fast forward to this weekend: I invited him and his adult daughter to holiday dinner Friday night but he only invited me (not my adult son who he is fairly close with), to his sister's home on Saturday night for her holiday dinner. When he first invited me I assumed my son was invited but he said "no". I met his sister once before and we certainly got along. Is this wrong? We are talking about living together, so why would she exclude my son?

Your son is an adult, your boyfriends sister is under absolutely no obligation to entertain any members of your family other than yourself just because you're her brother's partner. You say you've met his sister only once, so expecting her to invite other members of your family is unreasonable. You cannot expect his family members to feel obligated to include your ADULT son in everything. The clue is in the word ADULT. Your son will be sharing the Friday dinner at your place, why do you need him to be included in an event which is nothing to do with him? He should have his own social life going on. If he was 10 years old you would have reason to be questioning this, but as it is you're actually difficult and rude.