In a joint real estate venture what recourse do we have when one member stops making their share of the payments?

A joint venture is an agreement between to partners to share profits and losses and contribute to the venture equally/per the percentage ownership interests.  The remedy for breaching this agreement is for you to file a breach of contract action against the family member and lien whatever property they own (not the real estate venture asset, but individual assets).  The breach of contract action will certainly get the family members attention as you may sue them for the amount that they are failing to contribute. Make sure you have all the records for all payment you made.  checks are the best way to prove the damages here along with payment obligations of the property such as the mortgage an utilities. I would also hire a lawyer ASAP to make this less personal and more business-like.