This is a delicate situation, and I fail at tact often.
Our very dear friend M died 9 years ago this week, he committed suicide. He had schizophrenia, was an amazing artist, and left behind a very confused but religiously devout family (they are Jewish, M's father is a rabbi). We have kept in touch with M's parents, mostly, and speak with them at least a couple of times a year in social situations or through email/holiday cards. We are also very friendly with his sister (who lives out of state), but M's younger brother (who also lives out of state) has removed himself completely from anything regarding his parents. He believes his parents didn't do enough for M while he was ill. It's very sad, actually. The parents feel like they lost two sons. We have known the family for almost 20 years.
Which is why, when they asked to borrow a couple of M's paintings that M had given to us before he died for an art show of his work they organized at a local college, we wholeheartedly agreed. We lent them these paintings about 7 years ago, we got one back (and they generously framed it) but the other is still with them. As recently as last year, M's mother joked with me that she may not be able to part with it. That made me angry, but I can sympathize enough that I just laughed it off and changed the subject.
What would you do in my shoes? I want our painting back. Holidays are tough for me and so I spend time redecorating the house. It would be nice to see that painting on our wall again, but I don't know if I should ask for it back directly, let the whole thing go because of the family situation, wait a few more years, or something else.