Illumi left gradually. Spending more time with father and Killua first served training the heir, but after a while it was hard not to notice he started disappearing for different reasons. All his love is now directed to Killua, and all his actions are bound to him. Mother has to see his distance, but she knows it's necessary. The role of the firstborn is to take care of the younger siblings, and the heir is the most important.
Milluki left when he closed his doors and engrossed in self-indulgent fantasies. He doesn't have the proper appreciation for the family traditions, but his new technologies can serve it too, after all. Mother is accepting and she tries to understand it, even if this failure in his upbringing saddens her.
Killua left... many times. Each more painful than the previous one, but Mother's cry is always heart-tearing the same. And no matter how often he hurts her, she forgives and gives him her heart, just to be wounded again. When he's home, he has all her time and attention, when he's not home, he has all her thoughts and yearning.
Alluka left in early childhood, before fully becoming a person. He was taken away and replaced by the monster, and no one knows where did it come from. Maybe Mother still mourns the loss of her child or blames herself for that tragedy, but she never talks about it. That's the forbidden subject, and raising it can only upset her.
But Kalluto will never leave her. He has to be the one she can trust.
She doesn't even have the support of other adults. Grandfather is openly hostile and never tries to be more considerate toward her. Mother often recounts the inequities she has to endure, just for loving her children, and her voice becomes more shrill with every word, stinging his heart. Because of respect and fear, Kalluto can't hate his grandfather, but every time he sees him, his stomach scorches. He is almost grateful the man treats him like air anyway, so he can pretend they are strangers, only accidentally living in the same mansion.
Kalluto knows father doesn't appreciate her too. A long time ago he was hopeful when they kissed, because he thought it would make her happy, but now, he realized she deserves more. So he tries to be her consolation, be as elegant as she likes, be as beautiful as she likes.
And when she has an argument with father, she doesn't go to one of many guest rooms in the mansion. Quietly as a breeze, she slips to his room and lies next to him. He doesn't sleep, and he knows she knows he doesn't sleep, when she hugs him tight and whispers quivering words. Sensing her arm around him and her tears wetting his hair, he feels needed. And then he feels selfish, because she suffers, and he's thinking about himself. And he knows he will never leave her, because he cannot leave her.