If your twin sister is a sorceress, get her to cast Mage Armor on you. It's not really worth it to give anything up to get access to MA if someone else in the party already has it. (Though we tend to design characters in a vacuum, they're played in a group, so let that work for you where it can.)
Branching out a little bit, its never, ever, ever a good idea to give up caster levels but... take a look at a two level dip into Monk. You'll get to add your Wis bonus to AC, and for the second level bonus feat you can take Deflect Arrows which means once a round a missile attack that would hit you is swatted away. (If your casting, you'll probably have a hand free which is all this feat really requires.) For your 1st level bonus feat I'd suggest Improved Grapple, to give you a bit of an edge if you ever get grappled.
With a 16 DEX and a 16 WIS and Mage Armor, you've got a 20 AC off the bat. Shield of Faith and Owl's Wisdom is on your list, at least until you get items that do that, and Cat's Grace is on your twin sister's list. By the time you guys hit 5th level, you could have a AC 30 fully buffed. (Also, if you can finagle the Plant Domain that gives you Barkskin, which makes the Plant Domain worth it in my mind.)
Most of these spells are fairly short duration, but the Mage Armor is hours per level and the stat buffs are 10 minutes per level, and those should give you a +8 to your AC, on top of your DEX and WIS bonuses, so that puts you in the neighborhood of 24. Shield of Faith and Barkskin are another +2 or +3 apiece (I think these both bump up up to +3 at 6th). Items like Potions of Shield are also inexpensive and its +4 to your AC for 10 rounds.
Assuming you were fully buffed and CL 6 (I'd take Practiced Spellcaster, but that's me), it'd look something like:
Armor Class: 36, touch: 21 (29 vs incorporeals), flat-footed: 24 (+4 Armor, +4 Shield, +6 DEX, +6 WIS, +3 Deflection, +3 Natural). Granted, you'd probably never be able to get those three short duration buffs up, but really, how often does a 6th level character need a 36 AC? You get a Ring of Deflection or an Amulet of Natural Armor an that saves you having to cast the buff.
Branching out a little bit, its never, ever, ever a good idea to give up caster levels but... take a look at a two level dip into Monk. You'll get to add your Wis bonus to AC, and for the second level bonus feat you can take Deflect Arrows which means once a round a missile attack that would hit you is swatted away. (If your casting, you'll probably have a hand free which is all this feat really requires.) For your 1st level bonus feat I'd suggest Improved Grapple, to give you a bit of an edge if you ever get grappled.
With a 16 DEX and a 16 WIS and Mage Armor, you've got a 20 AC off the bat. Shield of Faith and Owl's Wisdom is on your list, at least until you get items that do that, and Cat's Grace is on your twin sister's list. By the time you guys hit 5th level, you could have a AC 30 fully buffed. (Also, if you can finagle the Plant Domain that gives you Barkskin, which makes the Plant Domain worth it in my mind.)
Most of these spells are fairly short duration, but the Mage Armor is hours per level and the stat buffs are 10 minutes per level, and those should give you a +8 to your AC, on top of your DEX and WIS bonuses, so that puts you in the neighborhood of 24. Shield of Faith and Barkskin are another +2 or +3 apiece (I think these both bump up up to +3 at 6th). Items like Potions of Shield are also inexpensive and its +4 to your AC for 10 rounds.
Assuming you were fully buffed and CL 6 (I'd take Practiced Spellcaster, but that's me), it'd look something like:
Armor Class: 36, touch: 21 (29 vs incorporeals), flat-footed: 24 (+4 Armor, +4 Shield, +6 DEX, +6 WIS, +3 Deflection, +3 Natural). Granted, you'd probably never be able to get those three short duration buffs up, but really, how often does a 6th level character need a 36 AC? You get a Ring of Deflection or an Amulet of Natural Armor an that saves you having to cast the buff.