How much should I charge for babysitting?

My brother and sister-in-law have asked me to take care of their 3 month old infant while they are in work instead of putting the baby into daycare. Because they are my family of course I want to help them out, but I would be watching the baby for 55-60 hours a week so would not be able to hold another job (except maybe for something very part-time on the weekends). I have my own family that I need to support and my brother has offered to pay me for my help. I'm not going to charge them the going rate in my area for a nanny (around $800-$1,000.00/week), but I'm wondering what a fair rate to charge would be. If I didn't need a job, I would help them out for free, but having children myself I know how much work an infant is and I do need some sort of monetary compensation. If I wasn't helping them out I would be working a full time job! What to do??