If you've ever done a repair like this in fiberglass, Now is the time to share your experience!
I know this can be done in fiberglass, seen it, but never done it. Check at the big box stores, they have a polyester wood repair kit that may be enough to do what you need done. Matching the wood and reassembling would be a challenge to any woodworker because those windows are built around the muntins. Muntins = the grid. Polyester resin, matte reinforcement and patience, Lots of sanding and shaping.. I believe a homeowner would have an easier time doing this than trying to make a replacement part from wood.
Personally, I'd build a mold the same shape as the outside of the window, just the depth and width of the divider between the glass, fold wax paper into it and shape the contours by hand to match closely, then when it sets, sand it and cut it to shape..