Remember that there is a reason why family members aren’t together... it was likely cheaper for them to get their tickets that way and they gambled on getting together once they were on the plane by getting others to move. This might be an issue if there were young kids in mind but with adults.... frankly, if I lived with someone 24/7, I think I might like to get some distance from them for a while and catch some sleep.
Anyway, here’s how I approached this situation one time....
A few years ago, I was on my way to Shanghai so it was a 15-hour flight. As I was stowing my bags in the overhead, I noticed that I would be sitting beside two cute Chinese kids... they were already buckled in and looked to be about 4 years old. I had the aisle seat and as I was still putting my bag in the overhead, a frantic woman was behind me very agitated and in half English, half Mandarin was loudly pleading “please, you must change seats. These are my children all by themselves. I must sit with them.” I paid no attention to her and finished putting my bags away and sat down. The level of panic quickly escalated but I paid no attention to it... She hit the point of hysteria and was wailing but I calmly showed her my boarding pass that indicated I was in the right seat and then crossed my arms and closed my eyes, all the time paying zero attention to the wild tapping on my shoulder. Finally, she ran away and dragged the head of the flight attendants to the seats. I proceeded to show the attendant my boarding pass and told her “Well, I don’t what the fuss is all about. I’m in the right seat and I’m quite happy here.” She says “well ok, but will you move if I can get an acceptable alternative seat?” I said “maybe but no promises since I rather like this seat.” She comes back a few minutes later and said “I got you another seat... you’ll like it.” I followed her into business class and took my seat which later became my bed which means it all turned out about exactly the way I was figuring it would... it is after all a long flight.....
A good story, skillfully told!
Regards,