Okay, some not-quite-list commentary first: Playing Sisters is a horrible way to get an affordable army
For all that I love mine, I can't currently recommend it to anyone who loves their wallet, especially as the squads as currently up on GWs site are not only pricey but usually also somewhat awkwardly armed.
Right, going down the list, Celestine. For your consideration: While it will WORK, I suspect putting the Fall of Cadia incarnation of Celestine in a 500 point army is a fantastic way to have people not want to play you anymore. She's ridiculously hard to kill at the points level. Something to keep in mind, especially for friendly games.
Note the Adepta Sororitas version is still legal (Codex: Imperial Agents does not replace Codex: Adepta Sororitas) if you love the model and want to field her. I've often put that version in a 500 point list without feeling any guilt about it, as it is rather easier to take down.
Battle Sister squads: Storm Bolters are rarely a good option. If you've got the Heavy Flamer in there, you're already committed to the short range, so grab a normal Flamer to go with it and burn all heretics extra crispy.
I'd actually recommend arming one squad with Meltaguns instead (and possibly swap the armament of an Immolator for the Twin-linked MM). Yes, Cadia!Celestine can handle vehicles just fine, but Meltaguns tend to work on everything... including those pesky people with a 2+ armour saves that actually take Celestine a while to hack through