I'm not sure I'd classify this as rules for the younger kids.
Firstly, the violin/flute/guitar are hers. You don't share smaller instrument, as they are personal, but she can be asked to keep them in her room. I'd suggest that anyway to avoid damage.
Piano, my temptation would be to tell him to move it to his house and the teacher comes there. You can get an electric piano that's more than good enough to practice on for both of them. They're smaller as well as volume controlled/headphones. Good news all round :)
However you have said that he isn't normally abusive, so I wonder if there may be more to it than you have said - or even that you realise.
Has your older daughter been stopped practicing because your younger one is using it?
Do the younger ones practice properly on it, or have they been using it as a bit of a fun toy and thumping on it?
If either of those may be true - or true enough your older dd may have complained about it, then it's not quite as clear cut.
In the former, I'd suggest that your older dd says when she wants to practice and your younger one does practice around her. She can't say "I want to practice now, get off" but she does get priority for the time she wants.
The latter, yes I'd agree with locking it. It can be unlocked for the practice time only and your younger one loses that privilege if she does start messing around. You may need to supervise, which tbf is a good idea at that stage anyway.
The other thing I'm just wondering is if there is any history of the older one getting/doing something and then the younger ones getting it off their back.
I was the older one who it felt like this happened a lot. I got/started something; younger brother had to have/do it too.
Often the item needed to do it, that I had bought myself or got for a present, I was expected to just let him have. When this had cost me up to 2 birthday and 2 Christmas presents to get I really resented him getting it for "free"!
I got nothing out of him using it, and he was generally less careful, so it often got dented/spoilt. If I complained, this was me being mean and not sharing. He never got int trouble, even when things were spoilt, nor was it ever replaced/mended.
He was never expected to share up to me with his things, so it was very one sided.
It was also often presented as "now you've got this, isn't it good - he can do it too" so I felt he was only able to do these things because I was providing them.
So if there is a danger that your older one feels like this, then I think there is a discussion that needs to be had. I'd agree that the piano should be shared however if she's feeling this way then you need to keep an eye on things and make sure that she does have things that are hers and she has her own space to do what she wants to do without feeling that her younger siblings will always be there too.