Not a lawyer, not your lawyer, but sometimes I watch Judge Judy.
I don't see why the brother should be forced to buy out the sister's half of the property. He is an owner of the property and is entitled to the rights of ownership, including the right to enjoy its use. The sister is entitled to the same rights. These rights do not include either depriving the other of his or her rights.
The sister has two options, in my mind: exercise her rights of ownership that don't require her brother's consent, or make a deal with the brother to do something more.
Certainly the sister can move back in and the brother can't prevent her from enjoying her share. The sister might even be able to rent or sell her share, depending on the kind of tenancy. If the brother lives there, the sister may need the brother's consent in the choice of lessee.
The sister can also try to strike a deal with the brother. Perhaps she could sell the brother her share, or buy the brother's share. The sister's rights here are those she would have negotiating to get a used car from a stranger after seeing an ad on Craigslist.