I agree she shouldn't have to share her things.
In my experience there are differences between older and younger from the starts, so possibly different strategies needed. My oldest feels inherently usurped by younger siblings and dislikes their cack-handed playing style that has sometimes broken beloved toys. He blossoms under a regime of having what's his, others not allowed to touch special things, no going in his bedroom.
My second child loves the older one and wants to be the same. So feels proud if any personal stuff is deemed worthy to be played with by older sibling! Therefore no need to promote sharing, as a go on the bike/ wear my jumper etc would always be offered (usually scornfully declined by older, I dont want that baby thing!)
We then praise younger for being kind (both in terms of sharing own things and NOT taking big brother's things).
As they get older, the older one also now gets praise for "letting it go" - You know it really doesn't matter if your brother uses something for 5 mins does it? Come and hang our with the adults, you're more like us than the little ones. That works for older now who gets "adult" treats like going to more adult places where the little ones can't come - we reinforce that's because he behaves more like an adult in his attitudes. But like an adult, he can rely on having agency over his own stuff.
At 2 and 4 though there's no way to know how they will pan out. Just give them both lots of reinforcement for both sharing and leaving the other to their privacy.
Finally - the other thing to reinforce now, as you get started, is the category of common resources. It is a huge world of pain when you have to buy 2 sets of felt tips, books, CDs, special individual footballs, blah blah...
We say, if it's something for all of us to use, all of us can use it. This goes for paper, paint, pens, a general large toy like big wooden train set, board games, food.... and clothes as soon as they NO LONGER FIT the original owner. Unless there is a special or rare thing like a special paint set, or something that gets used up like a colouring book, we hold as many things in common as possible.