It's true that Peter has had a tough time. Some of that is his own fault (not talking to Alan about his growing frustations), but like Alan, he thought that by keeping that sort of thing to himself he was sparing the other person. All of the recent dramas, however, are definitely not his fault, and he's struggling to cope.
Steve and Keidan will find things different with one or more kids around the house, but unlike Alan and Peter, they will know this before the kids show up. They'll have time to be prepared, or at least as prepared as they can be. I'm not sure, but I believe becoming a foster parent involves mandatory training, something most parents don't have.