Lease Pet Agreement/Disagreement

[Travis Guyer]

Hey everyone, my niece and sister are in a predicament and I'm not quite sure what the answer is:

So, my niece just signed a lease for an apartment and she apparently initially never told the management company that she had two cats and apparently the property manager never asked her if she had pets.

After a week or two I think she made it aware to the manager that she had two cats, so I think, the manager essentially said, "Ok" and sent her the pet agreement addendum which equates to $100 per month per pet extra in rent and a $450 non-refundable deposit. 

My niece said she didn't know about any extra rent for pets and told the manager that she can't afford an extra $200 per month and that even though the lease is signed she won't be able to move in.

So now the management company said that since she signed and my sister co-signed the year-long lease that she is still responsible to pay the rent for the year. My niece thinks she should just look for another place even though the lease is signed and she hasn't moved in yet, but she's worried about her mom's/my sister's credit taking a hit for cosigning and bailing.

I think there's fault on both sides, but I also think if they're not going to work with her on the pet agreement that she should just look for another place and just say screw the lease.  

What do you guys think?

Thanks!

Travis