well I don't wnat ot start a flame war but I like li'l sis
She can level up to a powerful mage and has all the basic thieving skills.
Also there is nobody you have to care so much about (except for Aerie maybe).
Hey it's older borthers/sisters duty to look after the younger, that's what my siblings did for me.
As for her banter. IMHO she sometimes takes you out of battle to a quiet peaceful place.