Answer: If both husband and wife were parties to the rental agreement for the apartment where they had lived together, they both remain legally responsible for the agreement. The husband remains legally liable for the rental obligations under the lease whether he physically moved out or not, unless the lease was amended to remove him.
Since he is financially responsible for the lease, under tenant-landlord rules, he should be able to enjoy its benefits, which includes the right to live there. There is no easy remedy available for your sister under tenant-landlord law, assuming your brother-in-law is unwilling to voluntarily amend the lease to remove himself as a party. Even if he was willing to be deleted from the lease — perhaps to end his financial responsibility — the landlord would also have to agree to amend.
Your sister may have a better remedy under family law. In the family law court, she could request a restraining order or other appropriate order in the divorce case that physically removes her husband from the apartment. The family court that hears the divorce case could also address the effect on your brother-in-law's financial liability if he is removed. If he is threatening to return immediately, then some type of immediate order from the family court would be the fastest way to resolve the rights of the husband and wife.
Eichner is director of Housing Counseling Programs for Project Sentinel, a mediation service based in Sunnyvale, Calif. To submit a question, go to http://www.housing.org.