If you were upfront about having pets before you moved in, it should be in your tenancy agreement and the landlord cannot change the terms of it without your agreement (this includes rent increases - not many people know this!).
If it wasnt mentioned and your tenancy agreement doesn't forbid pets, again the landlord cannot change the terms without your agreement. The only way the landlord can forbid pets is if it was stipulated in your tenancy agreement before you moved in.
That said, I undersand (as a tenant myself) that you want to stay on good terms with your landlord so I suggest offering her an extra 10% deposit and agreeing to have the carpets professionally cleaned and fumigated when you move out.
Keep things in perspective - yes, it's her property but you're paying (no doubt an extortionate amount) to live there. What if it were children your landlord suddenly objected to? Would she expect you to have them adopted?