I mean, with how his family has acted until this point I am surprised the others didn't consider the same conclusion as Klaus sooner upon hearing the story about his son.
The naivete, or stupidity, of this group is starting to get pretty impressive. The only way I could see them not doing so is that he is a relatively new character that none of them are particularly close or familiar with and that he may be trying to use them. I do understand that side of the argument.
However, I really dislike it when a character in a story is written to give people the "benefit of the doubt" despite there be obvious and serious character flaws in the suspected individual. Especially when the mc, or at least the main cast, is portrayed as calculating or intelligent (at least for the most part). Personally, I think it is a pathetic cop-out from authors. Despite what the totality of circumstances should have made obvious to the characters in their story, they still want to make the mc a cliched shounen "I will always believe in the best of people" type character either due to lack of writing or not having the sack to break from the norm.
This is how you write predictable and generic 6/10 to maaaaaybe 8/10 manga, not the 9/10+ that many love like Solo Leveling, HunterxHunter, One Piece, Marimashta Iruma-Kun (check it out), the Slime isekai, Berserk and others. Yes I know there are others worthy of that list and some maybe over-hyped. I just listed off a few from the top of my head that are for the most part of the same genre, or close.