I don't think Peter forgot anything while he was a rat, and certainly not the death of a parent. As for telling Sirius & Remus, when does he talk to them? I'm pretty certain we see all potentional conversations between them in PoA. Sirius & Remus ask him why he did it, and he gave a pretty direct answer-- he was afraid for his own life.
I don't believe there is canon information on Peter's dad, but the fact that his finger was given to his mother does not gaurantee that anything happened to the elder Mr. Pettigrew. Giving remains & a medal to a mother is a very emotionally evocative idea. There's something about the idea of a mother holding her baby that ties in so ironically to a mother holding her dead, adult son. It's entirely possible that McGonagall (I think it was McGonagall telling this part of the story.... maybe Flitwick?), or even JKR herself was aiming for that sentiment.