I was going to say "WHY is she pushing/ hitting", but you just answered that one. No apparent reason!
I have had a little dude like this. He would literally walk (beeline!!) across a room, just to hit a child who had done nothing at all to him.
I was on him like you-know-who-on-a-smartie. When playing, he was reminded what was expected. "We play nicely, and use gentle hands. Be kind to our friends."
Shadowing helped me stop him in the act a few times, but should he do it without me right beside him, it was an immediate time-out.
I think your biggest problem here is the fact that Dad thinks it's funny.
Parents need to be on board to curb the behaviour. Next time you talk to him (there WILL be a next time), and he brushes it off, you can remind him that just because a behaviour is "normal", does not make it OK. Yes, almost two year olds can be somewhat aggressive. But how on earth are they going to learn to change??? I think you need to make sure that Mom and Dad are willing to work on this WITH you.