Partnership Advice for a Family Business?

My Parents are selling the 4 of us their business (Me, Spouse, Sister and Brother-In-Law). We are each planning to buy in 50/50. Currently I have an existing business with my sister and husband. So just the 3 of us are partners and it works out great. I am concerned about making my brother in law a instant partner in the new business. I really have no choice. In order for my parents to Sell us theirs I have to divide it equally. I have no previous work history with him but know that he is not very motivated. My husband gave him his painting business when my husband partnered at our current business and He only works half the hours my husband did when he owned it. Since my sister will always vote with him and my husband will most likely see things my way is there any way to make it so that if we are ever deadlocked on a decision there is a tie breaker. Or if he elects to be lazy a way to vote him out later if he does not carry his weight with a 50/50 split? Thank YouPartnership Advice for a Family Business?
this is a tough problem ,if you have a good relationship with your parents visit with them about your concerns . it might not be worth losing your family over or maybe after you buy it you each could split it and go different ways without disrupting your current business with your sister .in any case i would not enter into any business with family without legal advice