I do like UP, and i don't think it and 'zero tolerance' of hurting each other are contradictory.
I have had exactly this problem, btw. She's very little. You are the parent and it's completely ok to physically stop her from hurting her sister. But do it in a , 'I can't let you do that, we don't hurt each other' with love, not pulling her away, or shutting her somewhere else.
Hovering is good, keeping communication open with her is good.
When does it arise? Any triggers?
Agree with lljkk about making time to hear her pov without judgement. Just hear her, ask her what would help. If she says 'send the baby back' as ds1 did, how about a HTTSKWL thing like, 'you wish she wasn't always here. Would you like us to have more time just you and me to do nice things together' and then when the baby's asleep DO IT, rather than catching up on housework etc.
Something that found massively helpful, esp when i was finding myself feeling v angry with ds1 was really remembering hard when he was a newborn, that giant overwhelming love I felt for him when he couldn't 'give anything back'. We had a small age gap, that 1st year was v hard for me, and this really helped, almost as a meditation, to get me back to loving him a lot. Also talking to him about what a lovely wonderful baby he was, funny pfb stories that made him feel really special.
The Sib Rivalry book by the HTTSKWL people is really good, have you seen it?