I don't have kids so take this or leave it...but is it fair to your daughter to move to MT? You said he is really good with her and he obviously loves her so do you really think he wouldn't have something to say about you moving so far away? Are you living in your sisters house? If so why not ask him to leave for a while? Not being a bitch here, but maybe he just needs to figure out the father thing for a while and just see the baby with out the stress of the relationship.

Hope everything works out OK for your daughter. Best of luck.