User blog comment:B214/Question & Answers 5/@comment-4547267-20150615043049/@comment-1359322-20150616030704

She's their master but that's all. She doesn't have the power to bend their will to her desire. If she does that she's no different than Diodora, and Ise himself would be displeased with Rias. So even if she manages to somehow block the girls from entering by giving them orders, she would cause some discord between Ise and herself.

Also seriously if Rias is capable of doing that she wouldn't have needed to argue with the girls so many times especially Akeno, the one she has least control on in the house.

P.S. the staying together is originally Sirzechs' suggestion and in fact order. So Rias has to obey her brother's order.