Well, if she didm't very much like her family in law, momcould be angry about him leaving. If she feels that Pria and herself should have had priority as his new family she could be angry. If she wants to stop Pria from going and she's despetate to do so she could pretend to be angry at him (emotional blackmail happens very often in these cases sadly), although I didn't picture her as doing that to that extreme from your description. I figured she would try a little, though. If Pria is all she has, it's natural to try and make her feel bad for leaving. Most of the time that kind of behaviour is done unconciously.

I was personally going for them getting pregnant not being planned and while her dad tried to do the right thing, he just found it too hard to have to settle down and lose his freedom like that. Whatever reason he would have to go off fighting a war, her mom saw it as an excuse to get away from a life he felt he was too young to have. In that case any excuse could do.