The family you go into business with isn't going to stay the same. Status quo for the family doesn't equal status quo for business. Long sickness, a new marriage, divorce, adorable babies -- all can affect the amount of time a family member has to contribute to the business.
When a family dynamic shifts, it's not just an issue of one person picking up the slack. Often in small family businesses, every member has an irreplaceable role to fill. Make sure that you have someone waiting in the wings (or at least a back-up plan) for life's inevitable personal hiccups.