Babysitting

This week I'm helping out my sister-in-law by babysitting two nights for her while she's working and hubby is away. I've watched her kids (my nephews) a few times for an hour or two here and there and it was fine. I've always refused payment because she's family and if its only a short time its alright with me. She tells me I should except payment because its her decision to go out and if I wasn't here she would pay somebody else. I compromised and told her if its an hour or two its ok. If its longer than that I'll accept her offer.
Yesterday I was babysitting from 2pm until 1am. I think the kids are more comfortable with me now which is good but one of the boys was off the wall crazy and refused to listen at all. There is a language barrier but we get around it with my limited French and the older child's help. It was a LONG day but after the mother called and spoke to the wild one he settled down a bit. When BF came over to help out he took charge which was a relief. He put them in the bath and then to bed. My hero. We are neighbours to them so it works out well.
Last night she paid me for yesterday and today. I will be working the same hours tonight. She asked me what my rate was and just said whatever you think is suitable. She handed me cash and I just put it in my pocket. I felt weird taking money from family. When I got home I counted it and she gave me $140 and I kinda felt bad. Should I give some back?
I guess when its family its complicated. We've had many discussions about this and she gets frustrated at me when I say I don't like to take money from her. That's why BF and I discussed it separately a few times and then the 4 of us talked about it once out in the yard when they tried to pay us for watching the kids for 2 hours one night. We told them we ate their ice cream and that was payment enough. We agreed though that we would not fight against taking money when we are there long hours.
So what do you think? Is it too much?