Just throwing this out there, but have you tried nursing in a sling or carrier to see if you can let your lo latch on or not as she prefers? That way you're not stressing about getting her nursed, your breast is right there if she decides to eat, and you can carry on taking care of the other child.
And as another sidenote that doesn't have anything to do with your question (so you can feel free to ignore it ), what happens if you try nursing her, she isn't interested and you just let it go for a little while longer?
she gets super distracted and arches away. she will suckle for let down, then gets distracted by the little boy and pushes me away.. meanwhile i'm leaking milk all over the place!
we used to use a sling when she was much younger, but she is never comfortable in them now!
great suggestion, though.