Use Hana's narrative. Draw on its strength since you have none of your own.
"Is Mari-sensei's influence over him greater than yours? You with the truth on your side and her with self-serving lies?"
Hana looked as if she had bitten into an acerbic tomato.
"And how would that be different from the power struggle you claim we're in?"
"Because you would not be lashing out against each other," Kei said simply. "Because you would not be attempting to force Hazō to sever ties with somebody he loves. You should not be threatened by a person you consider incapable of being his mother. On her part, Mari-sensei, or your vision of her, should know that tools can be shared as well as fought over.
"If I have learned anything over two years of bonding with strange people in the wilderness, it is that in a tight-knit group, each member is constantly influencing the personalities of the others. If you wish to join our tight-knit group, then you must accept that you will not be the only influence on Hazō, and that you will be influenced in return."
"Yes," Hana muttered to herself. "Please beat the career diplomat with arguments from human social behaviour."