Board Thread:General Discussion/@comment-25375901-20140918010005/@comment-4010415-20140918013253

Pretty much, Monty has said that Cinder is not Yang's or Blake's mother, and a few people on the RWBY crew have said that Cinder isn't too much older than the Beacon students. They said she's young enough to pass as a student. Consider the fact that students graduate from Beacon when they're 21 (They start at 17, and Ozpin said they'll attend Beacon for 4 years) and we can guess that Cinder is, at most, in her early to mid 20s.

Since Yang is 17, Cinder is too young to be her mother.