In this situation, the family memeber needs to realize that it's not really FAIR to ask for their children to be watched free of charge. There is a lot that goes into watching children, as she should know since she is a mother, & that would be on top of your own children. Just tell her that you can't do it for free. There is no way around it, you have to tell her & if you don't want to do it at all, just say no. My BF's gram watches our daughter (she's 2.5) for us while we work all day - but I tell you, it's not free at all & I would NEVER expect that! We pay her $300/month. Some ppl think that is a lot, but I don't think so at all, considering that she does everything for her. She provides food & drinks for her, bathes her, washes her clothes when she gets dirty or has an accident, makes sure she gets a nap, takes her out sometimes & buys her lil things, has bought toys & books for her to have over there so I didn't have to bring all of the ones from my house over & on top of that she watches her 40 + a week (often times she calls to tell me it's ok to work OT, wich is good b/c it's extra $$
), plus I know that she is in good hands & she learns a lot from her - she doesn't allow her to just sit around all day watching TV. She takes her out when it's nice out & they go for walks & all kinds of things.
But anyway, I just went way off... you just need to say no to her if you don't want to do it. It's not fair for her to expect you to just do it b/c you are a family member & for free non the less. I would never even think to ask someone that.