[MF] My girlfriend asked me to fuck her older hot thick MILF sister with a shitty sex life

So a bit of background on this, I’m 25 and my girlfriend is 24 her older sister is 33 and she always caught me eye, she has big tits and a huge ass and after her first baby not too long ago everything grew way more and she looks very thick. My girlfriend knows about what I thought about her sister and she was always cool about it.

One day I heard my girl talking to her in the phone and when they started to talk about her sex life she left the room to speak privately with her. When she returned she told that she had a proposition for me and that it would excite me. She told me her sister was going thru a rough moment in her sex life and felt ugly after her baby, needed somebody to take care of her in that sense because she didn’t want to go and date random dudes on tinder.