How much should I offer my sister for babysitting my kids (ages 1, 2)? How much per day?She asked me how much.

My lil' sis is watching my kids 2 days a week for me. (this is a temporary arrangement) probably for the next 4 weeks. Last week I was bringing take-out lunch from a nice deli for both of us to enjoy, each day. She has a 2 year old herself & is a stay-at-home mom. This kids play a lot in the large fenced in backyard & I WAS bringing supplies (clothing, food, diapers, wipes, etc) If I pay her xx dollars per day, do I quit bringing food? Her husband brought up this BURNING question saying that sis did not know how to ask me. I was annoyed that he brought it up, it should have been her to bring it up to me, after all, I thought I had a good relationship with my sis. So anyway HELP ME what do I do? I do not want to cause any bad feelings...