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