here is my take on it...

yes, I am a slumlord, IMHO you breached the contract when your parents move in.
when you apply for the apartment, landlord usually will have you put down who are the tenants, and each of them must sign the lease. so when your parents move in, you breached the lease.
so the landlord can ask for higher rent and back out of the promise for the free month rent.
your landlord can not do anything to you, but can evict your parent, so, you may want to renegotiate a new lease.