Board Thread:Speculation House/@comment-24.247.91.107-20140511154823/@comment-96.254.157.84-20140803005832

Summer is only Ruby's mother. Yang has a different mother, but they both share the same father. Monty confirmed it here: twitter.com/montyoum/status/492070189731565568

As for the last name thing, one of my theories was that in Remnant, chikdren take the surnames of their parents based on their gender (boys get the father's, and girls get the mother's) But again, this is simply a theory.