Bracing helps keep the joisting from twisting. Since it was built, it has twisted all it will. Make sure your plumber(s) haven't cut joists to install toilets, and things like that. Adhering sisters is a good idea along with structural screws, not drywall or brittle ones. Are you adding subflooring layer to what you have? Incidentally, what do you have for a subflooring? Thickness. 2x8's can span 11 1/2', but it depends on their age, condition and by sistering them, you are adding to that strength.