Ah yes, the "mine" phase. Geez, this parenting thing is full of fun stuff, isn't it
Just do as the pp have said, gently but firmly explain that No, that item belongs to (whoever) and tell her to give it to you, take it if necessary.
As for your breasts, dd went through this too and I had to explain to her that no, they were mommy's, even though she was still nursing. And occasionally now we have the "Yes I'm your mommy, but I'm also Gabriel and Cameron's mommy too."
You're going to feel like a broken record for a little while, but they do grow out of it. I promise.
What I found the hardest (and we still have occasional issues with it) was getting her to allow others to play with her toys. Especially if she's tired, she will sometimes get really upset if her little friends play with her toys. Now, if it's a special toy (like her stuffed kittys) I might help the other little one find something else, because those are her comfort items and it's not fair to make her part with them, but other stuff I will say to her that "Yes it's your toy, but you need to give your friend (name) a turn. When (name) is done, then you can play with it." And that usually means she'll now sit on my lap until she's over it, or so-and-so is done with the item, but she does get it.
Hope that helps.