Well, to figure out if it's your mother or sister in the manhwa raw, you need to pay attention to the details like their appearance, personality, and interactions with other characters. It might take some time to get a clear answer.
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.
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.
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.