This will impact your shear resistance, but it significantly depends on how much of the joist is cut away and how much is resting on the brick. If the roof is so steep that the entire width of the joist is supported several inches deep, it shouldn't have any impact at all. But if it's so shallow that the joist is only an inch or so wide over the support, then of course it will fail. Unfortunately I don't have the specifics on how much support you need (I'm not a structural engineer) but it wouldn't hurt to get the opinion of an expert on this.
I see in the bottom picture, the bottom is a tilt in sash, easy to remove. The top sash CAN be made a tilt in, it is missing the ballasts that keep it from dropping. It is now held in place by a metal or plastic clip that holds it in place. Find these, remove them and the sash will drop, and therefore, able to be removed. I see a screw visible near the tilt latch that may be one of the few to remove in order to drop the upper sash. Then again, that one may do nothing, I would expect to find the screws to lower the sash, under the upper sash, hidden by the lower sash. Remove the lower sash first the rest should go from there.
Most tilt in windows work the same, raise it a little, pull the clips in toward the center, lower the top so the plane of the window is perpendicular to it normal position, and lift one edge up, either the left or right, does not matter.
The rest is the same as the other answer, it will cost a little more maybe since you have the bars between the glass, then again....
Best Answer
tl;dr It's probably just a localized weakness and not a serious problem.
I suspect that the joist cracked due to 1) the off-color streak we see, which may be heartwood, and 2) the knot. Both are very weak points in the wood that wouldn't normally be a problem, but their unfortunate proximity combined to dramatically weaken the wood.
The outstanding question is whether there's undue stress on that particular joist, causing sag and severe tension on the bottom edge. We can't say much about that through this little hole in the internet.