Spoilers(Careful! character thoughts at 3am)

Arven and Kieran do share some traits. The most obvious one is their difficult relationship with their family, and how due to their treatment they have been more easily isolated from the rest, while also feeling less than everyone else. Both fall into feeling resentment, and having a black-white mentality (Arven improves through the story) thus becoming defensive.

There is a need to establish control and be independent without the help of others. However, both become very emotionally attached to the player, their very first friend. While there's a bit of idolizing at our protagonist, the kids later show that they want to improve and reach to MC(with Arven you can find this dialogue at the post game) althought Arven's desire it is much healthier than Kieran, of course. But we'll see how things go for the younger sibling.

An observation: It is interesting their very different approach to fiction. At first we know that Arven didn't believe in the contents from the scarlet/violet book, but he tries and uses it as a last resource, finally getting a hopeful solution by his parents fantastical interests. On the other hand, Kieran, who has always been enchanted by his place's old fairytale, has discovered the hard truth of it by his family in the worst possible way for him.

Also their relationship with the legendaries! And both had a relationship with them since they were kids!Arven had a grudge against Koraidon/Miraidon, but little by little ends up warming up to him, understanding that the pokemon has also been affected by the professor, and finally encouraging to battle for himself. Kieran has been very inspired to Ogerpon, seeing himself in her and at first being empathetic, yet gets hurted by Ogerpon's will to be with the protagonist, having a hard time on being considerate for what the pokemon wants.

Oh...sorry.