Maybe they were at the ATM and some sketchy people were hanging around. Big Brother felt worried about Little Sister's safety while they were waiting in line or during the transaction. And on top of that, the ATM itself had issues like not printing a receipt properly.
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.