That is quite a strawman, and you know it well. The problem isn't about characters being related or not, it's about getting new powers as the plot demands without any explanation or introduction whatsoever. Hopefully, you realise that there can be a middle ground between "fully explained in every detail imaginable" and "complete asspull", which is what we are being presented with.
Case in point, Sylvanas. Ok, we know she's an accomplished marksman, a formidable melee fighter, and has the ability to raise the dead. But all of a sudden, she kamehameha's Saurfang and uses a bunch of skills never seen before during the completely lopsided fight with the LK.
Where did all that stuff come from? "Oh it's the Jailor, guys" i.e. an "explanation" that doesn't actually explain anything. It's almost the same !@#$ they pulled in Cata with Thrall.