It's very adorable to see how much of a Doting Parent Viktor is with his children. He genuinely wants the best for them and has asked Heine to be entrusted to watch over and take care of his sons.
In general, the four princes are very good brothers to each other, with Kai looking out for the other three and stopping their fights, Bruno caring deeply about Licht's well-being despite their disagreements, Leonhardt looking up to his older brothers, and everyone wanting to help Leo and showing concern for him when he's having problems. This is especially poignant considering how common sibling rivalries and Aloof Big Brothers tend to be in anime and manga.
Manga and Anime
Heine reaching out to Leonhard and showing him that he is not like Leonhard's previous tutors who were constantly disappointed in him due to him being Book Dumb.
Heine: I shall teach him that teachers are not the enemy.
Heine's welcome party, full stop. It's even better in the anime where they combined the party chapter with the trip to the town. The stuff the princes bought became presents for Heine and the last scene of the episode show Heine in bed surrounded by the presents, that prevent him of getting stuck between the bed and the wall in a callback to the beginning of the episode.
In the same episode, Leonhart decides to pursue a thief himself because the mugged old lady reminded him of his own grandmother.
After a chapter of everything going wrong for her, Beatrix gets some happiness when Kai gifts her a flower crown and compliments her dress. Even better because she thought she looked ridiculous in said dress.
The princes and Heine doing their best to confort Adele when she is terrified of being sent to another country to get married. It's bittersweet because she'll have to leave someday but her brothers promise they'll always love and protect her
Bruno: Until I am certain that I can leave my beloved sister in his hands, no matter who the potential husband, I will be opposed to any and all marriage arrangements.
Licht: I can't have you be married off to just any old fellow either. He has to be at least as trustworthy as teach.
Kai: I think... you should be with whomever you want to be with.
Leonhart: You don't remember this, but years ago, when you were born, we were overjoyed. Father and Mother too. They looked so happy to meet their first daughter. All of us siblings, including our eldest brother celebrated together. We had a precious, adorable little sister... So we don't want to be apart from you either. But no matter how far apart we must be, we will always protect you
