I agree w/AnotherStander. speak to your lawyer about it as soon as possible. Hopefully it is not in spelled out in the paperwork. I agree w/you and your family, that if the project wasn't done that the money was earmarked for, then it should be returned to your parents.

You want to give this particular matter taken care of as soon as possible so that everyone is on the same page.

Sit quietly, the answers will reveal themselves when you least expect them to.
The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.