Maybe find acceptable outlets?

You could let her have a cushion to hit. Or maybe say you can see she has too much energy - how fast can she run around the garden? Time her. Ask if she can beat that time. Race with her - let her win too (at first at least). Wow, she beat Mummy! Looks like Mummy needs to practice more - want to go again? The speed toddlers run at over a distance, you can walk quickly with a sling on.

Time In is a big thing: do Time Out together when you talk about the feelings behind the behaviour and agree on what's not acceptable.

Agree that feeding her brother takes time away from "fun" things and you feel that pain too. But her brother has to be feed, he needs that. We can play later, but it's not fun when you're hungry, is it? When he's on solid food, would you like to help then? (I found with DS helping all of a sudden makes him happy and tidy and "pliable".)

Oh yes, and Nettie - just read your tagline. Your DH is right, I've been saying that for ages LMAO.