I would not use a single piece of heavier fabric, because this just moves the same stress from the seam and spreads it across the same fabric, same thickness, different spot.
What I would do is grade the material up. You need to spread the stress across the body with multiple layers of additional sil nylon. Since this is a corner? I would use circle shaped patches, cut away where they do not overlap any material. The first patch is the biggest, and against the body material. The second patch goes on top of the first patch but is about 25% smaller radius, so on and so forth.
This method will give you several rows of stitches, the material stretching slightly less and getting a bit stronger with each row and subsequent patch. A zig zag type stitch should be used around the arc part of the patches. This also gives you several rows of stitches spread over several inches of sil nylon. It would be quite a bit stronger doing this than relying on a single point of sil nylon to hold together.
Look at how grommets are sewn into the corners of sails. Similar concept. It will prevent flapping from destroying the seam too.