User blog comment:DiegoAnime3000/Why they have different last names? Answer found here./@comment-3146930-20140725032750

It is entirely possible that Long (or maybe Xiao Long, but Xiao could be a middle name) is the last name of the father and he was married to his wife and they had Yang. Then they split up or she was killed and he met Summer Rose. Summer Rose ended up getting pregnant within that first year or so of dating, but they broke up and Ruby Rose kept her mother's name.

A good example is actually my own family (I have a lot of half siblings). Anyway my mom dated this guy and she ended up getting pregnant. However my mom didn't really want to marry the guy, but she wanted to keep the baby. About 9 months later my sister was born and she has my mother's last name instead of her fathers.

Ruby could be the same way; she lived with her mother, but still visited her father every weekend or something. Meanwhile Yang is a result of his first marriage/relationship.