if i remember correctly she have been called slut in earlier chapters rumors must be gone to little sister school if big sister is slut little sister is to that's what people thought , so they are making fun of the little sister or boys trying to get laid with the little sister maybe that's why...
@siquall i am talking about https://mangadex.org/chapter/689194/9 this page mc hand is cut off properly going to do suicide sex or waifu angry maybe we have to wait and see