Legal Question in Family Law in Texas

Not in a divorce case. If he loaned you money and has a security interest in the house, I suppose he could foreclose on the loan and take the house from you that way.

In a divorce, he could claim that he has a reimbursement claim against your sister's separate property estate for money expended on increasing the value of the house. If amount of the reimbursement claim is limited to the increase in value to the home, not the amount spent. For example, if the house were worth $100,000, then he spent $30,000 putting in a pool, and now it's worth $110,000, then that $10,000 difference in value would be the limit of his reimbursement claim. If this is a relevant scenario, just know that there are also equitable offsets that you and your sister can claim.

Be prepared to prove to the court that you and your sister owned the house before marriage so that you can establish that it is NOT community property.

Good luck!!