help me with this landlord headache

Hi, there

I just moved in an apartment 9 days ago. As my parents are visiting me now, they have to live with me until they return to china at the end of June. My landlord has no moral or whatsoever in my situation. Basically, to get me sign the lease, she offered me a month rent free (for May). And now she wants to charge me the rent for may. Also, she is demanding a $200 dollars extra on top of the rent. But.

1. In the lease, there is no specific term about how long a visitor/visitors could stay

2. In the lease, she has specificed the rent.

I believe she has no rights to take back the free rent offer. I have no problem paying her #200 for May because my parents are staying. My question is

1. Would my parents' continuing stay would constitute a breach of the lease?

2. If so, what harm my landlord could do to me??

3. What is the best way now to protect myself?

Thank you very much for your advice.