How to repay your lover's sister Vietsub

On my way to my girlfriend's house, I met a girl. Because she couldn't call a taxi, she asked me to take her home, but I didn't expect that it was my girlfriend's older sister who had just returned home! After taking her there, she wanted to thank me in a foreigner's way. Because I didn't know what it was, I agreed, but I didn't expect her to thank me with … sex! I was tempted by her, she had a very big "soul", and allowed me not to use condoms, things my girlfriend could never do..