How much should I pay family to cat sit?

To accelerate paying off student loans, I’ve chosen to cut back in every category of my life. I used to casually drop $20-$40 whenever I wanted, and now I don’t spend on anything that is not a need.

My job is having me travel a few days and my sister (10 minutes away) will feed my cat while I’m gone. To make it easier I told her she could stay at my apartment, and her drive to work is the same distance as it is from her place.

I used to give anyone $20 a day to cat sit, and now being as frugal as possible, I want to be fair, but not generous (sadly. I look forward to when that can change.)

What is the right amount to pay her? She has no pets so I can’t ‘trade pet-sitting services’ - I would if I could!