How can she get her sister to stop guilting her?

“Since last year I’ve been watching my sister’s 9 year old daughter and lately she’s dropping her off all the time.
I told her that I wasn’t sure if I could keep watching her, and she told me that her daughter would be so upset if we stopped hanging out because she loves me so much and that I would be a bad sister if I didn’t help her out.
How can I get her to stop guilting me into watching her daughter?”
What do you think?