I Have A Theory

Percy didn't like this. He couldn't say anything without upsetting his fake sister more than he already had but it seemed like she was trying to take over Penny's life. 

She had never been interested in having pets. Even if Roman did say something about wanting one, there was no way she would agree so quickly. This was entirely that other woman's doing. 

Sure, it was a harmless enough action now, but what about when his sister came back and suddenly had to deal with having not one but two animals in her house? She had overstepped her bounds. That wasn't her call to make in someone else's life. 

Did she think she was stuck here forever? Was that what made her bold enough to change the way things were?

No. It wasn't possible. If they switched once, they could be switched back. She was NOT going to be here living his sister's life on a permanent basis.