User blog comment:Ajrep1994/Yang and Ruby are half sisters/@comment-3257939-20140723235141

I knew it! I knew when Lindsey said her parents, she was talking about Ruby and only her. I believe she meant that Summer Rose is Ruby's mother but not Yang's, instead, the only parent they share is dad.

That also works with my headcanon for their different surnames: After Summer's death, Ruby decided to take her mother's last name. Therefore, she is Ruby Xiao Long but decided to change it into Ruby Rose. Besides, Monty already explained that Remnant is not the same as our world so there is no telling that the children inherit their father's surnames and not mom's.