He is old enough to understand why he should share & if he’s in school kinder or first grade these are things that must be taught. I would make it a point that he is happy to use hers, but why doesn’t she have the same freedom with his? I would also let him pick out specific toys that are ONLY his and pick out ones he is willing to share. So he has a sense of “ mine” & has set boundaries.
He’s definitely old enough to be able to conceptualize why we share. I’d put the focus on if we don’t like sharing from a big huge pile of toys; let’s sit together & sort them! Into a pile of personal favorites; toys you wouldn’t mind sharing, toys you’d be ok with not being yours! Follow his lead at first, let the piles look disproportionate. At the end observer how many toys are in each pile & just work with him from there on trying to compromise which toys go into the sharing pile
This is harsh but I told my son none of the toys are “his” only. If there’s a few special ones he can put than to the side and they don’t go in the play room. I do give him the option to verbalize, “right now I want space/ want to play alone.” (He’s 4 and has a speech delay so this is something I’m actively teaching him). I think it’s healthy to play together and apart. So my question to you: is it he doesn’t want her to play? He’s playing? Is he afraid she’ll break it? Use it incorrectly? Or he won’t be able to use it if he wants to? It’s been helpful for me to know most of my son’s refusal comes from fear his baby brother will break it or, he won’t be able to use it when he wants it.
I think this is one of those times where you might want to focus on his emotional reaction to sharing instead of the reluctance to share. So for example, his sister is going to play with his toys. End of story. But that can be frustrating so how can he deal with those emotions? If that makes sense. Sharing doesn't come naturally to a lot of adults, forget the kids! So focusing on the big feelings of the "hard" fact could be helpful