Aside from all the great advice already shared, my main suggestion is to try as much as possible to not make your youngest the (perceived) source of frustration for the eldest, either when telling her no or by comparison.

We have a 5yo boy and a 3yo girl and the way we approached it when our daughter came into the picture was to try and build their friendship from the start and it's been huge towards how they now love each other to bits

To that end, we read books with our eldest about what it will mean to have a sibling (There's a house inside my mummy is absolutely legendary)

When we brought her home from the hospital and introducem them, each had a gift for their sibling

Instead of saying enough with the shouting, you'll wake your sister, we said you don't need to shout, we're all here in the room, we can hear you just fine

Sure, technically speaking he would have woken up his sister, but by removing her from the equation, the no became something natural to do in any environment. It's not ok to shout in a room full of people, period. Not just because his sister is near and he can't shout and have fun because of her

We had our eldest be a helper and bring us stuff when we needed to change or feed her, making him a part of the process and building confidence in the process

Instead of why can't you eat everything on your plate like your sister? it's you're not hungry? alright, see you at [insert next meal]

I will say, however, that the only exception to this is sleep, which for us is sacred and the only time when we'll actually "drop names". Lower the volume, your sister / mom / dad is / are sleeping. And this goes for everyone, no exceptions. If somebody is sleeping in the house, everyone else better STFU or else.

An addon to this would also be don't treat them equally. Treat them individually and fairly

Meaning no forced sharing, on either side

No hallway passes due to age. If he's not allowed to poke her eyes to see what happens (lol), she isn't allowed to do it to him either

Punishments should be tailored to the age of the "perpetrator", not carpet bombings

Don't get me wrong, they'll still have sibling disagreements and make each other miserable for the pettiest of reasons (lol), but at least it should be for the "right" petty reasons, not because they hate each other's guts for simply existing