How can I repair a fiberglass ladder?
Fixing the fiberglass is not a good idea. The point where the brace attaches to the ladder is frequently stressed in multiple directions and any fiberglass repair you do is likely to not hold up.

Fixing it with 2 steel plates is a better idea. If the damage to the fiberglass is small and isolated to just one face, then I recommend the steel be at least 6 inches long and as wide as possible. Use at least 6 bolts, if not 8.

However, if the damage extends past the face and on to the front or back face then I don't think repairing it is a safe idea.