Jan is very funny in general, in my opinion a must-keep.
Also, the plot drives you to rescue your sexy sibling, Imoen. Soooooo, I generally take her along for plot reasons (i.e. for my main character to have his wicked way with (kudos to the Imoen romance mod
)).
Aerie is a decent all-rounder, she is a good cleric and a good mage... but I usually have another druid/cleric in the party already... so she is out.
So basically, I'd keep two of them