Not ideal but normal IMO if the brother has enough income and actually move-in.
Few things to do/watch out:
1) the sister/mom is using the bother to get the rental property. The bother does not actually move in. Need to check all adults' background. make sure clean rental history.
2) the sister's potential boyfriend, ex-boyfriend, kid's daddy. Too many cases that single mom brings in someone landlord does not want.
3) the mom's ex...
4) This family is not a stable structure for ideal tenants. If you have no other choices, take them with extra deposit and careful background check.
Good luck!