well then that child wont be yours. you can chose to ether take care of it like a son or totally neglect it.
it wont be your responsibility.
how about after she has the baby, put it up for adoption.
you may be able to sue your father.
if my gf did this, i probebly would loose trust in her. it would take alot of time and effort from her to gain it back.
maybe she just doesnt care about this relationship enough. there are mistakes and then there are MISTAKES
and she made 2.
one was getting drunk and fucking your father.
two was not telling you about it.
so thus i would say....put it up for adoption. make it clear too. if she argues, tell her that she should have though about this before she screwed your father.