How much should I pay my sister-in-law to babysit?

I'm expecting a child in October. My schooling is an at-home type which means I can take care of her at all times. But sooner or later I plan on getting a part-time job to help make ends meet. Let's say I worked five days a week, and I had her watch my child during my work hours. How much should I pay her roughly? She lives right across the street from me, but she does have a kid of her own who is one. Any input will be appreciated. Thanks to anyone who replies.