'I suppose it's because you have different mothers' = which shows that this story was written before Togashi changed his mind about that.  ^_^   But, yes, his first idea was that Hiei and Yukina had different mothers. Here's the relevant passage:

Volume 6, page 115:
Hiei: Yukina doesn't know me. She doesn't need to, either. She's actually only my half-sister since our mothers are different. And there's a tradition that Ice Maidens don't leave the Glacier Country.