To my knowledge, roommates do not require documentation, neither do siblings, and frankly neither do business partners.
If they’re smart roommates get documentation. At the very least you have the lease agreement they both should be on if they’re renting together. And if one owns and is renting to the other one then they better have a lease agreement. Siblings have documentation, they both have birth certificates that show they have the same parents and what order they were born in. Business partners that didn’t bother to create an business or at least a contract are in for a world of hurt.