We had a widowed friend to dinner a few weeks ago and we know she and her mum, who is well over ninety, live together. She was describing how much fun they have together and how the arrangement has turned out even better than she had hoped. She put their success down to several things:
excellent communication
compromise
lots of good humour
having both shared and independent activities
and most importantly, they already had a very good relationship.
I have lived with my in-laws on two occasions; once for a year in their house and last summer for a couple of months when they moved into mine. In a few weeks time they'll be moving in again for a while as they are having building work done on their house. I love them to pieces and we have very complementary characters so we live together very harmoniously; I know that if it were to become a permanent situation it would work for us.