Board Thread:General Discussion/@comment-33053086-20171006004010/@comment-26397825-20171121225809

The Schnee semblance is as 73 said, unique in that they're not unique. Winter evens states so in V3: Lessons Learned:

Winter : We Schnees are unique. Unlike many, our  Semblance  is hereditary.

Whether this is the cause of magic or something else is unknown, though I'd honestly buy the idea that magic bloodline locked the Schnee semblance and made it so all future Schnee would have the originator's semblance.