I think 'a knowing sister manhwa hentai' could imply a manga that involves a sister character in a somewhat risque or adult-themed situation. But without more information, it's hard to say for sure.
It usually features explicit and adult-oriented content related to a sister character with some form of awareness or knowledge. But such content is often not appropriate and may be against moral and legal norms.
A sister-in-law referred to her husband's younger sister. Among the relatives, the husband's sister was called sister-in-law. In some places, she was also called a granny. His sister-in-law's partner was called his sister-in-law. The relationship between sister-in-law and sister-in-law was called sister-in-law.
Once upon a time, there was a little brother and a little sister. They lived in a small cottage by the woods. One day, they decided to go on an adventure in the forest. The little brother was brave and led the way, while the little sister was a bit scared but curious. As they walked deeper, they found a beautiful meadow filled with colorful flowers. They played there all day, making flower crowns for each other and chasing butterflies.
I'm not familiar with such content. Hentai manga often contains inappropriate or adult-oriented material that may not be suitable for general discussion.
It depends on how it's done. If it's just a simple pretend game where no one is made to feel uncomfortable and it's all very innocent, like just using a toy stethoscope to 'check' for pretend illnesses, it might be okay. But in general, it can be a tricky situation as it can easily cross into inappropriate behavior, so it's better to avoid it.
Another way is to make her write an apology letter if she has hurt someone's feelings. This helps her reflect on her actions. For instance, if she said something unkind to a friend, writing a sincere apology letter would make her realize the impact of her words and also improve her communication skills in the long run.
There could be a mysterious old neighbor. This neighbor might have some knowledge about magic or strange phenomena. When the little sister shrinks, the neighbor could offer some clues or tools to help her return to her normal size. Maybe the neighbor has had a similar experience in the past and can guide the little sister through her ordeal.