Assalamu alaykum Brother
I'm indeed very saddened to hear about your sisters situation.
Although it isn't very clear from your Question what exact you mean by sexual harassment. But such issues need immediate attention. No-one should be forced to adjust to something that Allah (azj) does not like.
I would suggest you ask your parents to speak to your Brother in law and also take some help from an Aalim or scholar and solve this for her.
I pray her issues are resolved. She is in my prayers