Your question is unclear to me. I gather (although I am not sure) that your in-laws will pay for an addition to your house. That addition will be used for
housing for some member of the family who needs care. You and your wife (I assume that is the "we") will be the main caregivers.
If these are the facts, I think that your in-laws have no authority to control, by their wills, ownership rights to the addition. Unless you have transferred some portion of legal ownership to your in-laws by a deed, you and your wife remain the sole owners of the house.
Your in-laws certainly can leave their property however they want it. If they loaned you money to make the addition, they can leave the loan note to whoever they wish to. Plus they could leave more money to other family members than to you (and your wife), to offset the amount they paid for the addition. What they cannot do by their wills is to leave part of your house to somebody else.