well, if i'm sabrina's boyfriend, father, husband or some other close relationship i will say, "mi sabrina", is somehow an expression that shows how important this very "sabrina" is to me upon any other "sabrina". It may seem like an expression to show that i'm her owner, but is not like that at all! However, it's more likely that here, the person is talking about his/her nephew (a girl, not a boy) and the word will be "sobrina". so if you are talking about you brother's/sister's son = sobrino so if you are talking about you brother's/sister's daughter = sobrina

I think that it should be "mi sobrina" that means "my nephew". "Sabrina" is a name but I don´t think it belongs to this sentence. Hope it helps.