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.