User blog comment:DiegoAnime3000/Why they have different last names? Answer found here./@comment-14640999-20140725044458/@comment-3257939-20140725051823

I think you missed something. I did mention that Ruby and Yang had two mothers. If Summer Rose is not Yang's mother which obviously means that her father had Yang with another woman, therefore, they are not full sisters since they don't share the same parents, only one.