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.
The wife's sister could be called sister, sister-in-law, or sister-in-law. The specific form of address depended on one's habits and traditional concepts. Some people might call her sister directly, while others might call her sister-in-law or sister-in-law. In addition, he could also address his wife as his sister. In short, this title was determined according to the individual and family situation. There were no fixed rules.
Sure. There was a story where the wife's sister helped the couple during a difficult financial time. She lent them money without hesitation and also gave them advice on how to manage their finances better. This act of kindness strengthened the relationship between all of them.