I read an article (I think it was on MDC) about how sometimes a child just needs to tantrum. And, sometimes when they give you an impossible answer... (Do you want your shoes on or off? NO!!!!) ...then the only answer is to step back and just give them the "permission" to lose it and tantrum.
So, you need to say, "I know you don't want me here right now, but in this family we treat each other kindly. Now, you need to give the toy back to your brother." Insert tantrum. Sit with her and wait it out. I'm always surprised that when I just open my arms at this point, my 3 year old will climb in them and let me hug her and rock her while she screams her head off. She'll usually keep saying how she doesn't want to give the toy back. And, I'll parrot back to her, how she really, really, REALLY doesn't want to give the toy back! And, how it's so sad that we don't have 2 toys so that they can both have one, but we don't. And, in this family, we are always kind to each other and we don't take things from other people. You wouldn't like it if he took the toy from you and I wouldn't let him do that, either. And, I know you're really, really, REALLY sad/mad about how she has to give the toy back. Etc.
These things can take awhile. But, in the end, IMO, you have to sit through it all and make her give the toy back. I think that's one of the most important steps. The message being, it's ok to feel the way you feel, but there are rules in our house and we all must follow them.