Was it merely because Sabrina hailed from a humble background and, in Kira's eyes, didn't deserve Tyrone?

Then why had Kira adopted Jennie, who came from a similar circumstance?

Tyrone had a strong desire to adopt Jennie. But Sabrina was hesitant, torn between her own desires and her respect for Kira's role in Jennie's life. They couldn't just take Jennie away from Kira, no matter how strong their desire to do so. Besides, they had only spent one month with the girl, and there was no guarantee that Jennie would want to stay with them.

However, recent events had shifted Sabrina's perspective, prompting her desire to keep Jennie.

If Kira could bruise Jennie’s arm just because the girl wanted to spend time with Sabrina, who knew what else Kira might do in the future?