I've been lucky both my wife and her sister like to be naked on the beach and in the sauna.
It took a couple of years before they went naked when my BIL or myself were present, but ever since we've seen each other naked on multiple occasions.
When comparing them, both my BIL and I came to the same conclusion.
Both women have the same shapes and curves, only the curves of my SIL are a bit bigger.
So their breast and nipple shape are similar, only my wife has a C-cup and my SIL a DD or E cup.
Also the lips of their pussy are similar shaped and both smooth in summer, trimmed in winter.
Finally their asses. Both still kinda firm and tight (for being 40 and 43 (SIL)), but again the one of my SIL somewhat bigger (which my BIL really likes).
No pics of SIL, both there are some around on this site of my wife, so you can have a fairly good idea of SIL
Maybe in about 5 years, I'll make another post to compare SIL and niece when she has grown into a young adult
