MC just nonchalantly blurts out that he was summoned with the "heroes", and everyone just nonchalantly accepts it just like that? No great reaction or several specifying questions about his origins?
This whole chapters feels like an afterthought. Like it was a bad idea which they just forgot to erase from existence.
MC has been very careful about his true identity so far, because he hates the thought of his sister finding him in that world.
Someone could think that him telling others about him being from another world, was a very stupid thing (me included), because that only adds to the odds of his sister finding out about his existence..
They are right. MC mentally "immature", e.g. naive, gullible, stupid. There was no advantage for him to tell them he was summoned to that world. He only puts himself at risk by doing that. What an idiot this guy is. It's like this protagonist lacks common sense, even from a modern perspective.