Bard. Have fun. still can heal outside of combat, plenty of options, boost and buff your mates.
ummm... what's needed is a serious face to keep your party from being lynched...
how about a div-spawn tiefling (+2dex/cha, -2 int; +2 diplomacy; misdirection as a SLA)?
you could go sorcerer and pick a couple of illusions as spells known to limit the chances of people realizing that one of you is an undead anti-paladin...
if you really want to heal, you could take 1 sorc level than 1 level of Oracle[wood] (with the Bend the Grain revelation) and then level Mystic Theurge from there... have a good amount of both arcane and divine spells, and be all Cha based so you can really crank your Bluff and Diplomacy.
edit: for MT, you actually need 2 levels of one of the classes, just to meet the skill rank requirement to get in...
also, bard is a pretty good fit too (Cha based, plenty of Face type class skills, some illusions for your own safety), the only real drawback is that they can't learn inflict spells so you can only heal half the party...
witches get cure and inflict spells but they're not Cha based and they don't really get any of the spells you'd need to hide the nature of your companions
Bard. Have fun. still can heal outside of combat, plenty of options, boost and buff your mates.
I'd agree with playing Bard. They can be a great class for mixing together such a weird group of PCs. Good support characters make for a great 'glue' to hold everyone together.
I second the bard suggestion. Bards make the best party face and are the optimal 5th man in a party.
You can handle out of combat healing with wands. The antipaladin should be able to cover any in combat healing that is needed, as he and the rogue will probably be taking the most hits.
This looks like a perfect slot for a witch. Some healing as well as some nice enchantment and necromantic spells.