This happened to me many years ago when me(m) and my cousin(f) were both 18. My uncle took us for a vacation to visit some family and we stayed with his mom, my cousins grandma.
My cousins grandmas house only had two guest bedrooms. My uncle took one and we decided that me and my cousin would share the other.
The house was a 50s style ranch that had a sunroom that had been added onto the back of the house, and the guest bedroom we were staying in had a small window that looked out into the sun room.
The second night we were staying there, her and I were up watching tv late while everyone else was asleep. She told me she was going to go get ready for bed and headed into the bedroom. Me being a teenager and super horny I thought I might be able to get a glimpse of her changing by looking through the little window in the sun room.
I snuck over to the sliding glass door and slid it open (pretty sure she heard this, as it wasn't as quiet as I had hoped), walked over and peeked into the window
She already had her jeans off and I got to watch her take her top off. she was standing there in a little blue bra and matching panties. she grabbed a towel and went into the bathroom to take a shower
I waited a little bit hoping to see more and she finally came back into the bedroom in only her towel still dripping wet...she was facing the window when she undid the towel and I got to see her full body. She had very round, perky breasts and her nipples were nice and hard. She also kept her pussy shaved nice and smooth.