Sorry if this is too long, this is my first time posting one of my past experiences, I hope you like it. Throwaway account as I wish to remain anonymous to casual viewers.
My GF (20F, let’s call her Sophie) and my GF’s straight sister (19F, let’s call her Poppy) are very close and have been eachothers best friends their whole life. They both look quite similar, with athletic bodies and are both quite short. Throughout our relationship, Poppy has always been really close and involved in our relationship to the point where we hang out a lot, have all see eachother naked before and talk about anything together. Me (20M, straight, in good shape) and Sophie are both really open about sex and love showing ourselves off when we’re drunk/high to our closest friends. Me Sophie and Poppy are all very close, and do MDMA together sometimes where we all become very ‘comfortable’ together.
The last few times where we have done md together, Sophie and Poppy usually like comparing their arses (as they’ve just started working out their bums) so they usually end up taking their trousers off and asking me to take a picture of their bums together so they can compare them.