Hmmm... That's interesting that you ask that.
Let's pretend that your brother or sister bit you: What would happen? Well for one, it would hurt. Secondly, it will leave marks on your skin from his/her teeth. The worst that could happen is the area that he/she bit you will get infected.
Now, let's pretend that a snake bit you: What would happen? Well some snakes have fangs (the poisonous ones). The fangs are long, sharp, and hollow. Just imagine those fangs breaking the surface of your skin, going deep inside your skin, and releasing venom (Keep in mind that venom is poisonous).
Now, compare the two. Snake bites can kill you, and they hurt worst than if another human bit you (depending on the type of snake). Not all snakes have fangs, only the poisonous ones.