Sisters Sabina and Lorraine Belkin get on sickeningly well: They live together, have the same friends, even go to the gym together. They also co-own Duo Restaurant & Lounge, New York City eatery. How do they keep their relationship healthy while working together in a notoriously stressful industry?

"We almost never argue in our personal or business relationships—we both rationalize the situation and come to a mutual agreement," says Sabina. Sounds simple—but how does that actually play out real life? I asked the Belkins, as well as brothers (and co-owners of New York City's Dos Toros Tacqueria) Leo and Oliver Kremer what they've learned about keeping it all in the family, while keeping the peace.

Have things in writing. "We didn't initially have a formal contract because we didn't have a clear view of what the responsibilities would be," says Leo. "After we opened, we created an operating agreement that spelled out financial issues more specifically, and what would happen in the event of a sale or bankruptcy."

Address problems immediately. "One of the biggest advantages of working with your brother is that we can hash out disagreements directly and don't have to be careful telling each other what we really think," says Leo. "There's a deep and implicit trust. Not that two non-brothers couldn't have that trust, but it’s almost automatic with family. Also, we crack each other up."

Know each other's strengths. "Lorraine is the social butterfly and loves to chat with patrons and clientele," says Sabina, who adds that her sister's experience with corporate and special events makes her the go-to person for those bookings. "She also went to FIT and has a great fashion sense, so she was able to help a lot with the design aspect of the restaurant." Sabina, in contrast, handles day-to-day operations, meetings, and negotiations.

Don't make it all business all the time. "We were hesitant to work together because there was a lot of concern that if things didn't work out it would damage our relationship," says Leo. "Our other worry was that we would talk less about our lives and relationship as brothers and would primarily relate in terms of our business. Luckily, none of this has come to pass!"