"he was living there with his mother, the owner of the trust, until she passed. His siblings did not live there"

Then he bought out the other owners at what got determined as FMV, right? Maybe it was at a discount (he lived there already, inconvenient for the others to try to market the property, etc), but that's not really a loss to him or the others.

It's a stretch to expect you could buy your own residence from your siblings at a reduced price and have everyone claim it as a tax loss.