The number of people upset about the older sister not being blood related is honestly pretty silly. I don't personally care about incest one way or the other, but really? It really makes you that upset that they're not actually related?
Kinda sad, honestly.
It's hyperbolic at best. Most are just disappointed with the tried-and-true copout that has plagued works with an initial incest premise. It has happened so often any anger that such a copout would have caused 15 years ago has completely melted away into being
completely expected. The state of non-H incest manga is a complete oxymoron!!! Brother/sister taboo romances actually ending in a taboo romance as they were advertised is seen as
a rare exception NOT the rule. You never see this with any other themed tag in manga, at least not to this scale.
If you go to a store and buy a product based on the packaging, you'd be pretty disappointed to learn that the product isn't anything like what it was advertised. Then imagine 60% of the products you buy from the store are plagued with false advertising! Even the bananas you'd buy would actually taste like blueberries! Wouldn't that be frustrating?
That's the same feeling incest fans get when they get baited. It has happened so often it's barely a tag anymore. Why even tease incest if it is simply not incestuous, you know?
All they can do is complain, say "well there goes another Non-Blood Related confirmation which completely destroys the premise that hooked me into this story to begin with. Oh well." and move on.