She’s a clone of Shmi. Snoke wants a vergence in the Force to birth Vader again so he brought back the Mother Mary character of Star Wars. Unsuitable clones are abandoned, not killed, in the off chance it might happen to them.
Snoke even chose various desert planets for the rejects in the hopes that he could recreate history.
Kind of like J.J. and his whole rehash of A New Hope. Ha.