I don't think the mother is the same. Where she find two different husbands (in a CDrama!!??)
I think he is either an illegitimate sibling or random cousin of the mother who is the legal/pure-blooded heir, but was disowned because she fell in love with a human.
They have to share a mom if he’s a pure blood and Xu’s father is a human bc he’s a half blood.
That would be the case if Xia Xu's mom was literally the only female vampire in existence... And she had baby with a male vampire first (aka De Yuan's dad), then eloped with some human and had a kid (aka Xia Xu's dad).
I'm saying De Yuan is a pure blooded relative of the mother (maternal cousin; in Asian countries, cousins are also called as brother/sister), not his actual blood-related brother, which would explain why he calls her "Xia Xu's mother" and not "Our mother" even though he calls Xia Xu "little brother".
I'm guessing De Yuan is simply lying about being his brother because he wants to get into Xia Xu's tablet.
Am I the only one that was getting weird vibes that De Yuan kept saying “your mother” instead of “our mother” considering that him and Xu are half brothers with the same mother…. Like something about it is off to me, or it could just be a translation thing idk