In order for me to help you answer this question, I would need more information. For example, were you involved in the purchase but couldn’t go on the loan due to credit issues. There are other possibilities that could be tied to this question.
By making the mortgage payments you have interest in the property unless you signed a lease agreement with them or rental agreement. If you did not have an any rental agreement, you should make some type of agreement where you have interest in the property. Also make sure you are added to the title. If you haven’t made any agreements or are on title, you need to make sure you have proof of making the mortgage payments. Make sure you get a copy of the cancelled checks from your bank and save them. Don’t pay in cash because it won’t have a paper trail to prove you have been making payments.
I hope your family is not driven by greed. I wouldn’t want to see a legal battle over any capital gains. I have seen families destroyed over money. It is sad, but true.