Axel would try his best to avoid talking about how envious he is towards his sister. He managed to get her a happy and healthy life by running away from the cult. She can grow up and have a normal childhood.
He never had any of that. He didn't have loving parents or made to feel special. He didn't have birthday parties with cake and presents while being surrounded by people who actually love you.
Axel hates feeling this jealous, it's no one's fault, especially Rosie. But that growing envy doesn't stop. She can go out and play, he can't without wanting to vomit. He hates this. He wants to be a normal kid, why didn't he get saved when he was younger?
Axel loves his sister, he would give her his life if he has to, but trying to not feel jealous is so hard. It's intoxicating.