Yesterday their was a news about rape and my FIL nonchalantly remarked that it must be the fault of the girl; girls should not venture out and be dressed properly. I really felt annoyed. How does one change people's mentality?

you don't and can't unfortunately change his mentality, but what you can do is definitely take a stand and be voluble about it. if you know that what he said was not right, why didn't you speak up then? or are you afraid of something? the only reason such people think like this is that no one bothers to tell them that the other point of view.

I do not think you can change someone's mentality and especially someone as old as your FIL. I suggest, do not waste your energy in thinking about changing your FIL's thought process. Do not have any conversation with him on topics like these in front or your kids lest they get to know that there exists a viewpoint like your FIL's too.

Don't bother...his age shows his mind is set. Don't waste good time in trying to change him. Instead if his conversation was heard by ur kids, try to adjust their thinking.