I'm not sure I'd be worried yet. Kids meet milestones at all different ages and if brother and mom are picking him up, it could just be a case of not learning yet or too lazy to do on own?

You've expressed concerns to mom, you encourage brother to let him be, you're encouraging child to learn on his Own....not much else you can do. Maybe set aside some time every day (when you're down on the floor with them maybe?) and practice standing with him. Eventually he will hopefully learn!!

Did the brother have this issue too?