Among what little Mara knows about Dorian is that he and his other two brothers lead three of the most important and reckless groups of the Clan.
But they are not part of the Clan, and only respond to the orders of the Clan Leader, to whom they swear to obey and protect.
When the previous leader passed away, they have to take the oath in front of her.
Although when they made the oath, she did not know about it, because at that moment she fainted and then several circumstances happened after that, and made it impossible for them to bring it to light.
That is why when little Mara heard Dorian said that he made the oath together with Ursula and the maids. Plus, he says that he wasn't the only one. It is an implication that his siblings also did it.
Little Mara has no memory of what happened after she fainted upon seeing her Mother in the state in which they found her in her room, but she knows for sure that it was something that would help her in the future.
**
