This was so badly written, I thought it was a dream sequence.
They're playing poker, a game from our world, but everyone there is proficient? Fine, maybe introduced by a previously summoned hero, but why does the deck look like a professionally printed deck, as if it was directly imported from earth?
They're drawing absurdly rare hands, all in the same round? Including the rarest of all, the royal flush?!
And, of course, what everyone's already mentioned: they completely skipped whatever conversation they had when he told them that yes, he was one of those summoned with the heroes. This CANNOT be news to them at this point, they cannot all have zero reaction to this bombshell, no matter how suspicious they were of his origin before.
But even FURTHER! They're saying "but who could the changeling be?!", to which he asserts "the younger brother"...and then they wonder what relation he, the fourth summonee, has to the rest...maybe that younger brother he mentioned? That he knows for certain is the changeling? Nobody connects the dots?
Then, as DeepBlu pointed out above, there's the Dinoso family...papa is famous enough he has to change his name to go into hiding, but his wife, "Ice Queen Shiva" is famous enough to be instantly recognized by name as well, but leaves her name in the open?
Ugh...I'm curious how the story goes, but not enough to put up with all these jarring jumps and wtf moments.