My GF (20F, let’s call her Sophie) and my GF’s straight sister (19F, let’s call her Katie) 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, Katie 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 Katie 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 Katie usually like comparing their arses (as they’ve just started working out their bums) so they usually end up taking their trousers off and revealing their perky bums in laced thongs and ask me to take a picture of their bums together so they can compare them.