My question involves landlord-tenant law in the State of: Iowa

My roommate and I need to terminate our lease early. Put simply, we aren't getting along and it is in our best interest not to live together anymore. I was able to move in with a friend right away but my roommate still occupies the apartment. We informed our landlord of our intent to terminate our lease early and he posted a listing on a local website seeking new tenants for our unit.
When I checked the website a few days ago, I noticed that the posting for someone to lease our apartment had been taken down; I assumed this was because he had found someone to take over our lease. When I called him to ask him what the status on the apartment was he informed me that he had taken down the internet advertisement because my roommate had called him and said that she could not find/afford anywhere else to live and had her brother move in to my old room (since I had moved out already) to help her out. Her brother is only looking for temporary housing because he's hoping to find a new job soon and move away.
In our lease, it clearly states that subletting without approval of management is prohibited. But yet the landlord is letting him stay there without letting me remove my name from the lease. Her brother (supposedly) does not intend to live there for very long (maybe 2 months) but we have 6 months left on the lease.
I was under the impression that we were looking for someone else to rent the unit and only found out otherwise when I phoned my landlord (shouldn't he have informed me of my roommates intentions? We are both on the lease, not just her). My roommate is the one who wants to stay. She has found another person to rent my room. My landlord is aware of this. What are my rights as a tenant to force my landlord to have my roommate and her brother sign the lease for the remainder of the term and have my name removed?
I feel like her brother will be able to leave whenever he wants and at that point I would be liable for rent again, which I will not be able to afford. Additionally, it seems as though the landlord is favoring her and trying to screw me over...