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?

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.