well first of all there are laws about what ages children can stay home alone and babysit so make sure that you are first obeying the law. In Maryland, a child can stay by themselves at 9 and then they can babysit at the age of 13. Some states do not have laws but guidelines and also the child protective services has guidelines of when they can step in.
I personally think that yes, the child must be paid to watch siblings. There are some things that are part of the family watching somebody else's kids for free is not one of them. That is just my opinion. If they say they will for free that is different. It is also different if like one is 14 and the other two are like 12, and 10. That is not babysitting that is just putting the oldest in charge. But having a child over 12 watch younger children is babysitting and should be paid.
Just my opinion.