User blog comment:DiegoAnime3000/Why they have different last names? Answer found here./@comment-3146930-20140725032750/@comment-3146930-20140725194026

But if her mother is dead and had died a long time ago, then Ruby and Yang would have been raised together starting at a young age. They are 15 and 17 after all, so even if Summer died when Ruby was 5-8, that would still mean that Yang and Ruby would have been together for about 7-10 years.