
I don't feel sad for the younger sister. From the beginning she wanted Helga to come back to her side. This kind of obsession is even worse than crown prince. He didn't push her. She on the other hand took the side of Helga's rival knowing they might spread her birth secret....she didn't care of what Helga wanted for herself. The younger realized that her house was a hell for Helga and she left because she didn't want it, she has absolutely no right to give it back to her. Can't she understand Helga cannot smile happily living in the same house as her mother? She blinded herself because she wanted Helga all to herself, by her side.
I won't feel sorry for her for what becomes of her.
It's not a spoiler but my speculation that Helga's rival lady is from noble faction who supports the empress. Empress doesn't want the crown prince to take the throne. They are plotting against the ML. And the younger sister is supporting them as one of the most powerful ducal household. Her mother shouldn't have brought her back.
(。・ω・。)ノ♡