I always find it problematic to be connected to someone both privately and professionally. Regardless of whether it's family or friendship, I would always be afraid that any private dispute would poison the working atmosphere and that any stress from work would be taken home with me. There may be people for whom it works well, but I don't like the idea. Apart from that, in such a constellation there is only one source of income in the household; if the company that belongs to both of them is doing badly, neither of them earns anything anymore...