If you want to play with Ember, you won’t want your PC to be a witch.
If you include Sosiel, you won’t want to play as an Oracle (they largely are duplicative roles). If you do NOT include Sosiel, you should play an Oracle because Daeran is evil, because divine spells are nice to have, and because channel energy is the most effective way to heal your party without constantly buying scrolls or potions and having to click through them.
If you include Ember, you’re missing some very good arcane spells (like haste/displacement). You can fix most of the gap by playing a Skald/Bard or by including Woljiff.
Alternatively, you could include a full arcane caster, like Nenio, or play as an arcanist. But if you do that…then why bring Ember. Some people love hexes. I personally think they are a micromanagement pain and not a particular good way to spend an action and don’t think she adds much, if you have a wizard/arcanist.
Since you’re set on Ember, I’d probably include Woljiff (who covers trickery, but is chaotic neutral, which might or might not be good enough) or include a Bard/Skald (who will fill the spell gaps and pick a background that gives trickery as a class skill). A bard will be better because you can spec into dexterity, which helps trickery, and be an offtank and inspire courage is usually better for a party that isn’t all melee.
So I’d have a party like this: Seelah (tank), Bard PC (offtank, dex focus), Sosiel (2h reach cleric w/ glaive), Ember, Lann, Arushalae.