Can a apartment complex withhold ur security deposit when u left because of health issues?

Bring the lease to local landlord-tenant counsel to find out his rights.

I'm sorry your father in law fell ill. Leases generally oblige the tenants to pay rent, even if they are ill or have a crisis that makes it difficult or impossible to pay the rent. The landlord is not only entitled to the security deposit for the first month of unpaid rent, but the landlord can also pursue collection of the remaining months on the lease term. You should try to get legal advice beyond this question forum. The landlord has an obligation to try to find a new tenant; many tenants who are facing the consequences of breaching a lease make sure they are monitoring the landlord's efforts, and even trying to help find a new tenant themselves. I wish you the best.