Callousness is the characteristic of being insensitive or hardhearted about other people's feelings. Your sister's callousness is clear when she tells you that your new haircut looks awful.
A callus, sometimes spelled callous, is a very hard, thick layer of skin that's usually found on the bottoms of your feet. When you act in a callous way, or exhibit your callousness, you could say you're "emotionally hardened." As early as 1670, this figurative meaning of callous was added to the literal definition. And callousness is a great word for the trait of being hardened against feeling empathy.