Going into business with friends or relatives is not a problem.
Just treat it like a business. When your cousin comes to work for you, you're under no different obligations as an employer than you would be if they weren't you cousin.
Actually, that's a real problem for most of us. A familial bond is one of care and protection. Family means that you defend other members of the family, even when they're stretching boundaries. And we have different levels of permission based on context, where the boundaries outside of the family are different than the boundaries inside the family. For example, if a kid gets into a schoolyard fight, the father might defend the kid's behavior; but if the same fight occurred between siblings, he might punish both equally.
A sociopath has no problem flipping the switch, to decide that they can ignore the family ties. For the rest of us, it's not that easy. (Please note that I'm not saying people who successfully hire and manage family members are sociopaths! I'm just saying it's hard.)
Looking at it another way, if it were "not a problem", if it was easy to treat family members equally, the phenomenon known as the 'Son of the Boss' wouldn't exist. But it exists everywhere.