Anyone have good advice on screening investment partners? I don't necessarily need a partner from heaven but I certainly don't want one from hell.

I'm currently experiencing a situation in where I've paid to setup an LLC, generated a business plan and even took days off work to make sure this joint venture is setup correctly.

The issue is my prospective partner has had bad partnerships in the past and now his mother in law, who is a retired mortgage loan officer, has a heavy influence on his decision making which impedes our progress. I don't really know them and they don't know me but the great thing about this business is networking and venture projects. I personally feel that I've invested too much time already and think he should probably straighten out his personal life before investing. A partnership is like a marriage. Must I be tied to his entire family.....?