She didn't want to let her go, that's why she had grasped her mother's dress with both her hands and had put he legs around her mum's. And yet Ygraine wanted to leave, she wanted to escape that house, leaving her little boy and girl behind.
Morgan cried, screamed for Arthur to help her and stop their mum, together, but her brother was standing emotionless near the kitchen's door: he was looking at them, his eyes empty and cold. He was staring at their mother taking his little sister's hands off her and head for the door not looking back.
Uther found them sitting on the stairs, Arthur gently patting his sister's back while she cried her eyes out asking for an explanation. The boy just stood up, hugged his father and told him he was going to put Morgan to bed; he wished him goodnight and picked Morgan up. She stopped crying at once, feeling her usually-teasing brother's strong arms scooping her up.
Well, if Arthur was going to take care of her, then nothing was going to be wrong.