If your sister does not need the apartment, she can try talking to the owner/landlord and telling them that she is getting divorced and has to move and ask if she can end the lease early. Some landlords will allow it, they may charge a fee. Others may not, and will try to keep you to the lease.
Your sister and her husband are on the lease, so if he leaves and doesn't pay, they will go after him and ruin his credit, but your sister's credit will also get ruined if she can't come up with all of the rent. And after that, they will require the co-signer to pay for the rest of the rent and lease. If he doesn't, they will ruin his credit too.
This is a very sad situation for your family. And you said that your sister was recently married. Yikes!!
I feel bad for your dad. I hope that the landlord/owner is understanding and that your dad ends up not having to owe anything.
I'd never co-sign for anyone.