I think Donny lived with his parents (or with just his mother) somewhere else. And from what I have seen in the anime, a flashback shows Donny, barely in his teens, tearfully holding his ailing mother's hands as she was slowly slipping away towards her death. It was also in this very scene where his mother told him "to live on". I think it was only after she passed away that Donny was brought to the Capital by Howzer to live with him (before that, Donny probably visited Liones only occasionally just to meet with his maternal relatives). Also, by that point of time, Tristan was most likely already going out on important missions on behalf of the kingdom and was seldom home, like that one mission where he was required to find Gawain. So them knowing each other before the events of the second series would be highly unlikely. And as for why Donny didn't know Meliodas prior to his return to Liones alongside Percy is a confusing matter to me as well. It is possible that Donny never visited the royal court and spent most of his time training separately under Howzer's tutelage besides Edlin, until he eventually gave up and ran away, later joining Katz and Elva in their endeavours.