Go off the Cape Morton reading if you are going to kite at Brighton. lol

The airport is only accurate in a Westerly.

I tend to go by the inner beacon more generally, sure its in the middle of the bay but does tend to give the more accurate reading. Then if its blowing from the north east go to the Sandgate pool as you tend to get more clean wind than Brighton. If its southeast then Brighton it is.

Good question. I've tried using all of the different beacons to gauge how windy it was at Brighton or other parts of the bay. After years of trying to use different beacons, now I mainly rely on the Brisbane airport reading and it hasn't let me down yet. A lot of people will say that it under reads the wind, but it's always within a couple of knots of Brighton in my experience.
A couple of things to note - the Brisbane airport reading is usually spot on in a N to NE. The reading will be gusty in a S to SE (assume it's somewhere between the average and gust speed), but still very close to the actual wind in Brighton.

I've been to Brighton a few times where it wasn't windy, but inner beacon is reading a steady 20kn. It seems like there's a lot of wind in the bay that never makes it to shore (or at least Brighton area).