I met my best friend and MOH in college. I went to school really far from home, and her parents sort of adopted me. I absolutely love her mom and dad and consider them like second parents. I planned to invite them. Her younger sister (age 16) is an amazing piano player, so I asked if she would be willing (I'm paying her) to play piano at our wedding. She was so excited. She also has a brother who is 19, but I don't know him because he goes to school far away. Since he is an adult, I figured I would be fine addressing the invitations to "Mr. and Mrs. MOH's Parents" and then writing their younger daughter beneath it. However, they recently adopted two young children.
Now I'm in a dilemma. I feel like I can't just invite the parents and then one of their children (two, if you count MOH, but she obviously will be getting her own invite with her husband). Since MOH's little sister is participating in the wedding and has already been asked, I can't just decide to only invite her parents. Do I now need to invite MOH's parents and all three children living at home? Should I then also send an invite to the son at college? I can't imagine he'd come, but it seems to rude to invite four out of five of their children.
I really don't want to add that many guests though, we're having a small wedding and are trying to keep the guest list under 90, which has been a huge challenge. Could I send the parents an invitation, and then the 16 year old her own, even though they live at the same address? Thoughts?