Legally speaking, the house is the property of your parents. If your father dies without a will, then his half of the property will be divided between your mother, you and your sister, that is, one person's 1/6. Family witness has no legal interest, because the property right gift is based on the premise of house transfer. Without house transfer and written agreement, she can go back on her word at any time. Then, three years ago, the house 1/6 was yours, and 4/6 was yours.
Your mother died a year ago, so the part that belongs to her is shared by you and your sister. Of course, you can share more by taking care of your parents.
So in theory, it is of legal significance for your sister to divide the property, but interestingly, if she filed a lawsuit with you before 1 year, it is better to say that it is cheaper for your brother-in-law to file a lawsuit now. Because they want a divorce, if your sister doesn't want a house under the pretext of divorce, you can give him a good explanation:
Legally speaking, the house is half of your sister's, but without the transfer, she can't actually get the house, which means she hasn't inherited the house for the time being, so she can share their own savings when divorcing your brother-in-law, and then share the house after the divorce, which means your sister gets half of the house. Now that she wants a divorce, she has to go through this procedure with you first and give it to her ex-husband for no reason.