I need her help
At the start of the week, when Paul went on duty, I explained to Sarah that I was going to need her help with a lot more things, because the Littles are so sick with a stomach virus and they'd need my attention. The problem is she can't do what I need her to do. Like she'll pick up the baby and bounce her and make her scream while I'm telling her she needs to be fed.
What would you do if Sarah wasn't around?
Well, you'd figure something out because it was a necessity!
What I'm seeing in your interactions is Sarah trying to do something to help and what you want is conscripted labor.
Or another way of looking at it is that you're trying to split yourself in two and have Sarah act as your clone and do everything you would do if you could be two people.
So I think you have to let go of the control. If Sarah wasn't there the baby would cry while you helped Logan. Or Logan would throw up on his own. Or you'd hold the baby the best you could while helping your son.
Ask her for whatever help she can give, offer some suggestions and then let go. You could say, "Would you mind doing something with the baby? Maybe you could try giving her a bottle, since that sounds like her hungry cry to me," then let go of control. (That'll be the hard part!) And when the crisis is over say "Thanks ever so much!"
You're thinking in the moment. You want the right kind of help right now. But that type of reaction takes withdrawals from the relationship that you pay for in the future: a child who is less willing to help because she feels that what she can do is unappreciated.
Maybe by her age she should be feeling compassion and wanting to jump in to help, but she's been set back because she feels what she can do isn't wanted or appreciated. Or maybe she's still in the development phase of not being able to see with compassion. All she sees is a mom whose taken on more kids than she can handle and is dumping the excess on her.
Maybe it would help to reread what you wrote and substitute yourself in for Sarah and your husband in for yourself. Imagine that he's doing something that seems totally ridiculous to you, like juggling flaming torches and wants you to handle one set in a particular way while he handles the ones that are giving him trouble. That's quite likely the way she sees it until she can get to the point where she knows whatever help she can do will be appreciated.
I read Joyce's [above] advice. I agree with it as a thought, but I know it's frustrating to need another pair of hands and someone standing right there, and not helping.
Yes! I was trying not to say don't even think of asking her to help, while also trying to convey the idea that her help shouldn't be seen as something that can't be done without because if she wasn't there, mom would find a way to handle it all alone.
Another pair of hands is a bonus. An extra. Something that needs cultivated and appreciated. :-)