I can't believe I haven't replied to this thread yet!
I have two sister-in-laws.
One of them I never really talk about. We get along great, but she doesn't interest me in any way. She is "conventionally attractive", extremely outgoing, wears way too much make-up, DDD natural boobs on a tiny frame, we get along very well, but she is from an extremely rural community in western North Dakota, so culturally we clash quite a bit. I haven't seen her naked, but I have seen her in a full-coverage swimsuit that outlined her enormous nipples (not areola, but actually her enormous nipples), and the conservative swimsuit even outlined every detail of her folds down below. But, it did nothing for me; nothing at all. It's fun to know those details, but I honestly couldn't care less whatsoever. She gives great hugs, but that's about it.
My other sister-in-law is the one I talk about all the time. She is the most confusing person on the planet. (Well, I know her inconsistent signals and behavior is due to extreme jealously, resentment, and her own miserable life, but it's still confusing in the moment as things happen.) She is absolutely stunning to me, although not conventionally attractive, she never wears make-up, she has never dyed her hair, she is naturally very fit and pleasing to look at. She is extremely awkward (yay) and embarrassing in public (nay), and she is absolutely batshit crazy and could use therapy about her rough past.
Anyway, I have seen all of her bits and pieces here and there over the years, but not all at once. I would love to be casually nude with her, either in a hot tub, changing at a hotel, or whatever.
I see down her shirts and into her oversized bra every time I see her. She has tiny, droopy boobs with extremely long nips that point up. Sometimes she knows that I am looking and she smiles and doesn't care. Other times she puts he hand over her shirt like I am the creepiest person around. One time her husband caught me looking and told her, and she flat out said, "I don't care at all that he looks," and pulled her shirt outward so I could see inside as I was standing right next to her when her husband mentioned it. But then later she acts upset.
Same thing in skirts. One time she got up from a chair while I was on the floor with a baby and she smiled at me, opened her legs, and slowly stood up. Another time she fell, legs wide open, and I was going to help her up, but she said she needed a moment as the fall hurt, keeping her legs wide open. Other days she acts like I am, again, super creepy when she is wearing a skirt.
I have sort of seen her topless, but only her back, and once barely a side-boob, and one time she was facing me when she was trying to clothes she bought at the store, but I didn't turn around in time, so I didn't see get to see anything. I would love to see her casually standing up topless someday. At a cabin a few years ago, she was changing in her room and someone opened the door, and all 12 people there saw her topless from the living room except me because I was in the kitchen. I was so bummed that I missed the one chance and everyone else got to see her. My wife said that her sister didn't even rush to cover up! I was bummed for months for missing that!
I have seen her absolutely gigantic bush. She was squatting in a dress to pick something up outside, and pulled her dress above her knees to not get it in the dirt. She was facing me at an angle perfectly so that I could see into her loose panties and it was a bush bigger than in silly movies when they glue on a bush the size of Texas. I admit that was not a satisfying moment to see. Nope. Hopefully never again. (The other times I have seen up her skirt I didn't look that closely, as I am not an upskirt person anyway.)
There have been times when we have been alone and she has cuddled with me even with her husband next to her. We were walking our babies once and she grabbed my hand while it was on the stroller, then she realized what she was doing and freaked out and walked 30 feet ahead of me the rest of the time. Another time she sat next to me outside and cuddled with me, but as soon as she heard voices of our family, she got up and sat far away from me.
One morning at a hotel I was staring at her long nips poke through her pj top and she smiled at me and arched her back and let me look.
Another time she saw me looking and she covered up and put on three layers.
She has seen me naked dozens of times over the years, sometimes staring and other times acting like she is freaked out.
So, as my one sister in law is totally normal, I feel indifferent about her, but because my other sister-in-law is so over-the-top bizarre and inconsistent, the mystery has intrigued me waaaaaay too much over the years, obviously.
I have fantasies that my crazy sister in law leaves her dead stump of a husband and moves in with my wife and me so we three stay together, etc. That sounds like fun in theory! (But in reality it would be a complete disaster--she can be really vicious, mean, controlling, and probably abusive for all I know.)