User blog comment:DiegoAnime3000/Why they have different last names? Answer found here./@comment-69.117.13.82-20140725043949/@comment-3257939-20140725060642

Wow, you really took your time to think this through. I'm glad so I'll do my best in this one. Ok, here goes my answer:

The naming tradition was just the way I came up with an explanation that can support my idea since it's very simple and we don't know if Yang got her last name from her father or her biological mother.

I also noticed that fact, when they talk about the mother, they always make sure to refer to her as Ruby's but when they talked about dad, they made sure to put a resemblance between that man and Yang by making Ruby compare them.

I also agree with that. If the man is not really her parent's brother, then, Uncle is used to refer to a friend of the family. But I've never heard of someone calling an older guy "uncle" if he has no relation to the family. Meaning that it's more likely that Qrow is an acquaintance of Ruby's parents.

I agree with the idea of leaving the 2 years difference between Ruby and Yang as something irrelevant, I don't believe the exact amount of time can make a difference. It's the fact that it counts. This is a short period of time taking in count that it take nine months of pregancy so it rejects a lot of other ideas about what happened.

Unlikely. It seems that Ruby's father is a good man since he taught her to always help other and try to make friends. If he can raise these two girls who are kind hearted, I doubt he is enough of a douchebag to have children with two women without their consent.

That leaves only your ideas for this one. Something must have happened right after Yang's birth. I also agree that the most common idea is that Yang's mother died from birth or that she decided to abandon her child and her husband right after having Yang but that would mean that the relationship between Yang's parents was already at stake before her birth. If she decided to abandon Yang, that could also be a good support for another headcanon: The woman Yang was looking for is her mother. But that is only speculation for another topic. Yang has been described by the writers as a party girl at first glance but in the inside, she's a caring sister who would do anything for her little sis in a heartbeat, why is that? I bet it's because after Summer's death, like Monty said, Yang had to be there a lot for Ruby and they grew even closer. So I guess Yang decided to take Summer's place and take a good care of Ruby. This could mean that Yang did have a good relationship with Summer, Ruby is not really her full relative but so it's not Summer, I doubt she would have a rocky relationship with her since, like you said, she was at least 1 year old at that time. Not enough age to start hating someone since you have no conscience. And even more since she took care of Yang when she was just a baby. She would obviously grew attached to her so the only reason why they wouldn't be as close as Ruby was with her is Summer herself. Again, like you said, maybe because she wasn't her child. The only reason why I don't buy that last statement is only my own opinion: I doubt that the mother of a very nice and kind hearted child, such as Ruby, is capable of holding Yang to a lower degree for not being her biological child.

That is also my answer for your idea. I don't have anything to reject it but my own opinion again: I really really REALLY with all my heart, don't believe that such good kids could have been raised by parents with a lot of problems such as cheating her with another woman, besides, like you said already, it's frankly terrible writing.

Yes, that one contains of holes, therefore, it's even more unlikely.

Finally, my statement remains, I do believe that we have enough info to believe that Ruby changed her last name to fit her mother since she loved her so much and was crushed by her death. The only thing we can't explain is where Yang got her last name from? But it's very obvious that the four theories you said are almost the same, the only difference being the motive of why they got the last names. At the end, the answer is still simple: Yang's mother or her father have the last name Xiao Long.