Thank you.pretty much. A babies born from the timing of the womans egg and mans sperm cells, so it just happened this time round draka and durotan timed it differently and had a daughter instead of a son.
But yeah, this creepy weird shit of claiming its not incest, consider this fact, the alternative that is Wod occured from the point in time of when garrosh travelled back and made the alternative reality occur, durotan and draka were still alive then, which means its only because of that moment in time with garrosh occuring this new timeline happened. Thrall still is the son of these two even if they didn't give birth to him in an alternative reality, so Geya'rah is still his sister.
But then, given the fetish this fanbase has for undead slappers like Sylvanas its no surprise they don't consider their opinions weird.