Baby, I don't agree with Nemo/Kesha's advice at all. This is not something that you should be having to deal with. If in fact you do want this situation, please find someone you can talk with about it even if it is a school counselor/youth group leader/ parent/ aunt or uncle, anyone you trust.
i think this person may be setting you up to be with her son - because like i said earlier...if her son is your age, then why would she ask you to come babysit someone your age? Why would he not be able to babysit the younger sibling?