User blog:DiegoAnime3000/Why they have different last names? Answer found here.

Let me explain this the best way I could find:

We just got a new confirmation from Monty about Ruby and Yang being half-sisters. Now, don’t confuse this with step-sisters. A step-family is someone who becomes a member of your family but has no blood relation with you or whatsoever. On the other side, half-family is a member of your family who is related to you by blood. If Ruby and Yang are half-sisters, then, that leaves only one explanation: The share only one parent.

Since Summer Rose has been referred multiple times as Ruby’s mother, it’s a major hint that Summer has no relation to Yang, meaning that the parent they share is not the mother but the father. The problem is that Ruby is younger that Yang, so that means that after Yang was born, her father lost her marriage with Yang’s mother. And since humans spend nine months inside the belly before being born, that means that right after Yang was born, something happened which ended in Yang’s father leaving behind her original wife and marrying Summer Rose and this happened in less than two years. Whatever happened after that we don’t know, but if Ruby was so sad because of her mother’s death, she must have meet her which means that they spend a lot of time together as a family. One of the first things Ruby said to Ozpin when he asked why she wants to be a huntress is that her parents taught her to help others.

The next topic is the reason for the last names: Ruby Rose and Yang Xiao Long. This one is tricky because we don’t know how naming works in Remnant. One of the possibilities is that in Remnant, children could not be named after their parents like it happens in most of the countries in our world, there is always a chance that in Remnant or even only Vytal specifically: Children inherit the last name of the parent with the same gender. Why not the mother or the father?

The mother contradicts Jaune’s last name. He is an Arc and apparently the members of his family who were huntsman are only males. Not women. If Jaune said that the other Arc of his family, who were heroes and all that, are only male and not female. That leaves me with only one explanation and we go back to my theory: He is Jaune Arc like his father, grandfather, his father before him because they are all males so they do share the last name of the parent who had the same gender as them.

The only real problem with this theory is Weiss Schnee. She has the same last name of the proud Schnee family of the Schnee Dust Company. And it has been stated already that her father and her grandfather are the ones who run the company. However, it has never been proved that these two have Schnee as their last name but since it was her grandfather the one who established the company: It’s impossible that the company was named after the wife since the other Schnee of the family is Weiss’ father who is male. UNLESS, Weiss’ father is not a Schnee but the husband of the daughter of Weiss’ grandfather, in other words, Weiss’ mother is the real Schnee who got her last name from her own mother… wait NO! That contradicts everything. It’s impossible for Weiss’ mother to be the real Schnee since the other Schnee is her father who is male and she’s female. That means that they don’t get the last name from their mothers or the person of the same gender, leaving only one option: Children in Remnant (or Vytal) inherit their last names from their fathers.

So to put it simple, I came up with this idea precisely to break it and leave only one single explanation to the question we are all wondering about: Why they have different last names?

It’s very simple indeed:

<span lang="EN-US" style="font-family:"Arial","sans-serif"; mso-ansi-language:EN-US">Yang got her last name, Xiao Long, from her father and so did Ruby. However, after Summer’s death, Ruby chose to change her own name and have her mother’s last name, therefore, Ruby Rose. And that’s why they have different last names.

<p class="MsoNormal"><span lang="EN-US" style="font-family:"Arial","sans-serif"; mso-ansi-language:EN-US">I spend a lot of time in this theory so I’m very proud of it and confident that is right. Any contradictions or loops that you found would really help me to work on this so any answers are always welcomed.