I'm going to give the sister the benefit of doubt, in earlier chapters the sister was so dismissal about relationships stem out of love I think it has to do with this blondie. She's overly obsessed with making money and looking after her family she doesn't believe in feelings that lasts. And Dahae is living carefree life wrapped in fantasy. It might also be the reason she was kinda testing Beom seok if his feelings for Dahae is legit or not that's why she offered to initiate kiss after knowing Dahae was into him. Because Beom Seok had a series of dating incident that time.
I'm going to give the sister the benefit of doubt, in earlier chapters the sister was so dismissal about relationships stem out of love I think it has to do with this blondie. She's overly obsessed with making money and looking after her family she doesn't believe in feelings that lasts. And Dahae is living carefree life wrapped in fantasy. It might also be the reason she was kinda testing Beom seok if his feelings for Dahae is legit or not that's why she offered to initiate kiss after knowing Dahae was into him. Because Beom Seok had a series of dating incident that time.