Ahhhhh. Welcome to the trials and tribulations of
FAMILY business. The key to our surviving for 120 years through 5 generations has been keeping business and personal completely separate. Not just money, but also lives. Heck, the wives wouldn't even allow the men to discuss work outside of work. What happened between 7-5 stayed there from the time they left at 5 until they got in the next morning at 7 (which basically allowed a cool down period).
It sounds as if you're comfortable with your cousin's (?) abilities and with a partnership it's not a big deal. But should you grow into a LLC or Corp., and bring more family members in, be aware that everyone else (non-family employees) will see any deficiencies that someone with the same last name has. We've got a requirement that any family member wishing to work for the company start working at 16-17yo after school hours, doing all of the menial tasks (making boxes, sweeping, pulling items for packaging), then during summer break, learning every part of the business as they become more competent. After graduating college, they're required to work for another company (or themselves) for at least one year. If they still wish to work for the company, they've fulfilled all of the requirements, they're qualified and there is a position available, then they're in.
That generally takes care of any family member that thinks they're entitled just because of their last name and probably not a good fit. There have been many bitter cousins over the past century+ who thought they were just going to slide right in, and quite honestly, it was obvious that they were not good fits early on.
Any way, good luck!