As title.
Happens On family hols, when staying over at relatives houses and over many years. They choose this sleeping arrangement.
Grandson used to spend a lot of time with grand parents when small and it seems to have just been a sleeping arrangement that has kept on as he has got bigger and bigger. Grandmother is in the picture and chooses to sleep on sofa or air bed.
Aibu - this is really weird behaviour and should not be normalised and it's OK for me to refuse to go on hols with them or allow my kids to stay over as they obviously don't have normal boundaries
Yabu-this is withing the realm of normal family relationships and get over it