Your sister’s portion of the house will pass according to the terms of her will or under the laws of intestacy if she didn’t have a will. Unless your parents had set up a testamentary trust, your sister’s share was hers to do with what she pleased. Just as your share is yours to do with as you please. As joint owners of a house, each owner has the right to use and occupy the house. It is very likely that your brother-in-law now has an interest in the house, but you will need to consult with an attorney. A probate will need to be opened for your sister’s estate to clear the title of the house and determine the need owners.