why is mojej used here as opposed to moja?
The window belongs to your sister - therefore it is in genitive (possessive) case? That is my guess.
I thought it meant: "THIS window, sis?" Did Marmaduke steal the pie from THIS window?
Could i say "to jest mojej siostry okno"
In poetry maybe... generally it sounds rather strange.
Can i say: "This is the window of my sisters." ?
Sorry... Of my Sister!
Our native expert told us that such a construction is really rather too unusual in English.