Murtagh and Eragon accidentally find a small puppy and decide to adopt it. The doggy stays on Mount Arngor with Eragon, and whenever Murtagh visits them, puppy follows Murtagh around. When Eragon and Murtagh are studying together, the dog is lying under the table between them

"Revel in your newfound family, son of Morzan. It is the only boon you are likely to receive in this sorry world"

The Twins were right. Even before Murtagh found out the truth, Eragon was his guiding star, Eragon became the center of Murtagh's world. Murtagh literally planned to attack Galbatorix himself, with no magic, no dragons, a suicidal desire to show that he is more than his father's son. And the way Eragon so unexpectedly came into Murtagh's life, they became friends and for Eragon, Murtagh would have done anything. He wasn't fighting against someone anymore, he was fighting for someone and it was for Eragon. At some point, Eragon was Murtagh's weakness

So yeah, Twins were right that Eragon was Murtagh's only boon, even before they were brothers

"Revel in your newfound family, son of Morzan. It is the only boon you are likely to receive in this sorry world"