Keep in mind Alucard is basically a teenager who just killed his father, whom he loved and was lonely. He wasn't exactly as vigilant as he probably should have been and he paid dearly for it.
No, what was stupid how the twins went about the whole thing. I got the idea that, because Alucard wouldn't tell them how to move Castlevania, that meant that Alucard had somehow betrayed them. The idea is laughable, especially when one considers the idea that Alucard may not even have any idea whatsoever in how to even fix the castle's ability to move anywhere in a blink.
That might be also the reason why the twins were really insistent in knowing how to move the castle. They had to get home with their new knowledge "now", not in however long it took them to get to Alucard from Japan (at minimum, considering the timeframe they are living in, I expect it to have taken at least a year at best...it might have taken them longer.)
They might have managed it faster if they got on the Silk Road, and managed to brave their way through the challenges there...which might explain what they were talking about when they mention how they got "good" at betraying their potential enemies on the road.
Still doesn't excuse them, though I do pity them for ending up as a grim warning for anyone who might approach the home of the heir of Vlad the Impaler.