Tenant wants his brother to move in

[Nicole Heasley Beitenman]

A young couple is applying to rent my investment property. One of them asked if their brother could also live there. We don't have a problem with that, but we obviously would like him to be contractually obligated to follow the rules of the lease even if he isn't financially obligated to pay rent to me. We also want to do a background check on them. Is there anything else we're overlooking, and has anyone ever dealt with a similar arrangement? 

TIA!