It's considerate to watch someone's kids.
Sally's sister asks her if she can babysit their children that evening so that her sister can go out.
Sally wants to give her sister an answer regarding Sally babysitting their children.
Sally says she's happy to watch her sister's children to let her sister go out and enjoy herself.
Sally says that she doesn't want to take on the responsibility of looking after her sister's children.
Sally's sister is able to go out and feel secure that her children are being looked after by someone she trusts.
Sally's sister has to pay for an expensive babysitter and worries about leaving her kids with a stranger.