Brother, sister and girlfriend threesome

I was walking on the street with my girlfriend and my sister when a guy came to us and asked us if we wanted to earn some easy money doing a threesome. For me and my girlfriend it wasn't a problem but doing it with my sister? First I didn't know if my sister was up to it and second, I didn't know if my girlfriend would be ok with it too. We talked for awhile and decided not to do it but at the last time the guy offered us more money and that, beside our doubts, made us say yes. My sister is the black haired girl. At first it was a bit weird but it came out ok, I think every one liked.