There are Federal laws that govern watching children in your home. It's called the Child Care Facility Licensing Act. Please call your child care licensing agency and see if what your state laws are regarding this before you take the chance of getting huge fines for running an unlicensed child care facility. Some states allow you to watch one or two children without having to be licensed. It's not hard to get a license either if you decided to do that.
I would also call around to see what home day cares are charging. If she is lower income she could qualify for subsidized child care expenses and only pay a small co-pay. If you are licensed and get a contract to receive those funds you could make much more money and she could pay MUCH less.
Also, if you get licensed you can count off a percent of your house payment, utilities, phone, buying toys and supplies, etc...on your taxes. They become business expenses.
But please take this as just a "watch out for yourself" kind of note. If even one of your neighbors call it in that you are watching kids and aren't licensed the state can descend on you like a flood. People watch their friends kids all the time, switching out for an occasional evening out, etc...but watching a child on a scheduled basis is child care not just babysitting.