I will sometimes take Dark Angel Allies for my AA. 1 Tac squad and a Nephilim Fighter. If 1750, I might through in an Aegis Defense line with lascannon (and have the Dark Angels man it).
But my Exocists are great AA. Sure I need a 6 to hit, but with D6 shots and 2-3 Exorcists...always, I have a decent chance. And at Str 8 AP1 all I need is 1-2 hits and most Flyers go down.
You can also use the Forgeworld Avenger Fighter...but should we ever get a full Codex and Army redo, I'm sure a Sister's only Flyer will be included.
Perhaps the new I.G. codex will have an Air Superiority Flyer that will make a great Ally unit.