Ah yes, if it's an ear, then the arm does need to go behind. Also, if you continue the bottom line of the ear to where it meets the head, it's going to be below her jaw. If it's cat/animal ears, it needs to go even further up on the head than normal human ears, otherwise it looks wrong (unless it's elephant ears, which also come off the sides of the head. Oh, and arm/ear... one should be casting some sort of shadow on the other, so it's easier to tell one is behind the other.
And fes is right, some stretching of the shirt would pull it away from her chest, also probably put vertical creases in it. (But now I'm just nitpicking.)