@kowoyoshi I wouldn't say you are wrong, but not correct either. I usually don't try to argue against comments like yours, but just two remarks.
Tl;DR - Arein and Arjen are too distrusting of family, but there is some reason to it. Also, it is probably the author's intension to do so.
One, they do trust their siblings somewhat. Arien and Arjen are people that only see the worse in others. The Princes all knew of their existence, but not once met them before. The 2nd prince himself said that he "
wasn't interested" (Ch 8) of them until they met. Speaking of the 2nd prince, while he did use magic to protect them, he also has been hostile with them. When they first met he threatened them because he didn't know who they were. Then in Chapter 7 after seeing Arien's scars, he forcibly grabbed her. I can't blame the 2nd prince, he is only 10 years old, so he wouldn't have the best control over his emotions nor be that caring of a person he hasn't met before. So while they can understand that their brothers are trying to care for them, they also see it as them doing it on a whim. Basically, "They care for us right now, but that can change at any time." The Third Prince is similar as well. He is very friendly with them, and they recognize that, but they were told and saw how violent the Third Prince could be. They think that if they ruin his mood at any point, he will get violent towards them like he was towards those noble children (Ch16). Though the biggest reason why they don't trust the Emperor and the Princes is because of chapter 32. The Emperor saying that if it wasn't for the contract, they Arien and Arjen wouldn't be alive, and for him to send them back to the old castle. That is basically saying that everything that has happened up to that point was for nothing. Chapter 35 defiantly made cracks in the distrust for Arien, but Arjen is far more stubborn to change than she is. Likely because he lumps all the members of the royal family together and not seeing them as individuals. Another thing that modern people often do.
Two, I agree with you on some points, in that they shouldn't have been so trusting of the Duke's son and Riolu, especially the Duke's son. I sort of get them trusting Riolu since they have similar backgrounds, but they are really naïve. They really take after the Emperor in that way, in that they believe what others tell them without checking first. The Emperor believes that they hate him, and Arjen and Arein believe that the Emperor doesn't really care for them. Both could be solved with a conversation, but it doesn't since they already have it set in their minds. It is a kind of "Why check on something that I already believe to be true?" situation. It is a very human type of thought. I mean how often do people repeat false information because they believe it to be true?
The situation is not stupid because the story is stupid, it is because people are idiots. The Twins trust the Duke's son because he tells them what they believe is already true, and the Princes only contradict what they believe. They trust Riolu because they believe them to be alike. They twins lost trust in the Duke's son in their last meeting, "
He said he would be our friend , but he mocked us instead" (Ch 36). They distrust family because of their mother, but want to latch onto someone. So, a friend and a friend that is in a similar situation to them would be best. Hence why they want to trust Riolu.
Sorry if my comment is a bit inconsistent. I was typing it as I reread the series. I reread not only to build my argument, but to also see the validity of your comment. As I said in the beginning, you aren't wrong that they are too trusting of nonfamily and too distrusting of Family. But I think that was because the author wanted it to be so. Of course you are allowed to have your opinion as I have mine. I am mostly writing this not to change your opinion, but to provide a different angle. And maybe stop some people from dropping the series after seeing your comment.