All Perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as that of his family and all his companions.
If your husband wishes to live in this house and his sister moves out when she marries, then she has the right in her share in the rent of the house, which is according to her share in the original inheritance. Your husband is obliged to pay this share to her unless she renounces her share in the rent or renounces her share in the original inheritance. Anyway, whatever you agree upon should be documented in order to avoid any disputes in the future.
However, if this sister wants to take the price of her share and it is not possible to divide the house, then everybody is obliged to sell the house and then give her share to her. Nonetheless, your husband and his mother may pay to her the amount of her share in the house after which she would have no right asking for this house to be sold.