Her feelings for sister exist. She could have decided to move on, to accept, to calm down, you name it. However, it doesn’t change the fact that she loves another person, while “starting to have feelings for Kiku”. That is bad storytelling. Because her feelings for sister are now absolutely useless information, not telling anything and not helping anyhow. They only explain her depression, which could be explained by more realistic things like injury or social issues. In other words, she had a problem. She didn’t solve it. She accepted it and moved on to having romantic life. Two almost unrelated story parts, one not influencing another.
The way I see it - the theme of incest became uncomfortable and the author decided to give Sena another love interest. The way she is now, she will always have silent unrequited love inside. Which is sad, really. Because it seems no one will be happy in this series. I feel tragedy, not romance, and it’s confusing.