>>715498025
She is onee-san.

>>712550725
In that scene (it's from a comedy anthology manga series) she and everyone else are basically drunk. They tried to get a sleeping potion for her to help her sleep better but it got splashed everywhere. So while everyone else started acting goofy, she basically reverted back to how she was before all the shit happened to her and started mommying/big sistering everyone.
She did end up getting a good night's sleep though.