Nina acts as the older sister, despite being the younger twin, and refers to Sara as "Onee-san". She's overprotective of Sara and worries about her well-being. She wants Sara to be with her "predestined person" (Mei) and tries to get them together, also trying to keep Yuzu from getting in their love's way.
She was very thankful for returning her sister to her and they became friends but was angry at Yuzu for trying to get in the way of Sara's and Mei's date. She knew of Yuzu's feeling towards Mei's but still chose her sisters happines over her friendship with Yuzu. it gets resolved in the end and eventually they have a good relationship.
She seems to like Mei only because Sara believed she was her predestined person and was hoping to bring them together however it fails at the end. But she see's her now as a good friend to sister.