They didn't create those kids! If you can't afford to pay a babysitter w/o relying on your present kids, maybe you shouldn't have more kids until you have that figured out. That makes me so angry seeing a child's childhood snatched because their little lives are wrapped up in their siblings. It isn't right!
Uhh because it is free. Because she isn't doing anything anyway. Because it is one day and it won't kill them to spend time with each other. Because I rather trust her with her sister than some stranger. Those seem like good enough reasons
I think if your having your older child watch your younger ones for hours on end, day after day then yes it's wrong. But a few hours every now and then i see no problem. Usually if my older child watches the younger ones i will pay her what i would a babysitter.