It's probably a bay window.
Here my mother sat in her bedroom window and wrote in her diary, “I am sixteen today. How old that is!”
How can a person sit in a window? Does it make sense? I think "by" would be a better word here.
What is the opinion of a native teacher?
Even if it's not, you will still hear people say that they are sitting "in" the window when they just mean that they are sitting indoors in front of the glass, as if the window were a frame. This is the Google Image hit page for "sitting in the window": https://www.google.co.uk/search?hl=e...mg.WKwp05NRG2k
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.