Nope, you are not alone.
When I first played BG1(English version), I never once thought she was your sister.
I actually thought she would be your romantic interest, like Sarevok and Tomoko.
Of course, after hearing about BG2: SoA, I got it and was surprised that she suddenly became your sister.
Yes, since she was my favorite character in BG1.I am curious: would everyone still have found Imoen so endearing if she had kept her [ugly] BG1 portrait?
Her new portrait was a definite plus though.
I don't think you should do that, freedom may be nice, but that won't follow the story lines at all.This is the reason I intend to give the player a lot of choices. Not everyone thinks about Imoen in the same way. Lock her in the cage she frees you from for all I care (and if that can be implemented, it will be).
Considering that it is necessary for her to get out of the dungeon safely for the game to advance.
Of course, after you rescue her, anything goes.
Because really, you may not even plan to rescue her all along and just wanted power, which would be the second reason why you chase Irenicus, according to the storyline that is.