I never knew Imoen was your sister. I'm only up to chapter three though so...... When were you informed of her being your sister anyway.

you will be told by Irenicus.

I also said when was I told

I think it's kind of touched on in the opening of the game...or maybe I just read it on these boards...but both Imoen and the protagonist (that would be you) are children of a now-dead evil god who walked the earth and impregnated mortal women. Apparently, this was something you learned at the end of BG1, but I never finished that game. So you and Imoen are half-sibs. Half demi-god sibs, to be exact, which may explain why the protagonist, no matter what class you pick, has special abilities like Champion's Strength and Larloch's Minor Drain, because you have the blood of a god in your veins. (At least both my protagonists had those powers.) This is that secret potential and secret power Irenicus keeps hinting at in your dreams.