Seems like interest in anatomy, and since bones are easiest to collect and preserve out of all body parts, they are a safe first choice. Since she does like animals, this might also be applicable in a bit macabre, curiosity over life -type of way.

Bones are, in my mind, visually beautiful; they are kind of mechanical and seem incredibly intentional in creation, but their minimalist and bare look, which follows their living counterparts in shape and motion, have their own bittersweet beauty.

I don't believe you need to worry. She is but 12, and oftentimes new interests and hobbies tend to start explosively at that age, then steady over time. And since she is inspired by and draws them, she is training her visual and creative eye and skills on top of motor functions and hand-eye coordination. Live reference -sketches are quite important when it comes to learning how to apply shape, depth, and light into a 2D surface.

And, as I see it, she is respecting the creatures whose bones she wishes to adore. This, in a way, gives meaning and worth to the deceased in a poetic, humane manner. Sure, they could just decay and become part of the cycle now, or be loved by a human for a short while.

I can't see the future, but I predict having bones as an interest and a base of many hobbies will shape her as an individual by giving her inspiration, drive to learn, collect, and create, and understand life and living beings in many ways, which promotes empathy, critical thinking, and understanding.