The word “sill” usually means the bottom part, but in this case you seem to be referring to a leak at the top of the window, so let’s call that the “head” of the window.

If it leaks when you run the kitchen sink water, it could be what your roofing contractor says. It leaks when it rains, you have rainwater, and need to focus your attention on the area above the window on the exterior.

Can you send me one picture of the entire window from the inside, and one showing the entire side of the house where the basement window is, all the way up to the roof, and one more close up at the exterior top of the basement window? Send to Mark@ChicagoWindowExpert.com and we will get it fixed!