your parents need to look at their lease. It will state their how long "guest" can visit (over nights). If you exceed that amount and the landlord finds out, he can give your parents a warning and tell them their guest must move out or they can either raise their rent or evict them.
I know first hand as this just happened to me. My name is only on the lease. The landlord knows my son lives with me (hes 3) in a 1brd. My brother stays periodically to help with his nephew. My place has a garage so i let my brother store some tools because its safer and close to him.
My landlord is under the impression my brother lives here because hes always here and I have some of his belongings in the garage. He told me they feel this is the case and in August my rent will go up $500 and a new lease must be signed. Its legal here in California to do that. Because I breached my lease due to the occupants in the unit (they are not listed as living there). However...in my case, this is all based on an assumption. Therefore no action was taken as I made it VERY clear to them, their assumptions don't justify a rent increase or a new lease.
Again, have your parents look over their lease about "guests"
hope this helps!