R.I.P. :(

I'll reiterate, for BG1 you could go with almost any party composition. You won't handicap yourself, not without purposefully making dumb choices in char development / equipment / world actions.
If however you're planning on continue playing the same char in BG2 then go with a fighter / mage multi. Be sure to roll high too.

What mage (not) to take with you through the course of BG1 is up to you really.

With all that said, I'm still not sure why you won't just go with a "canonical" good party and be done with it.

BTW Edwin is not out of the question per se, but hauling him around in a good party is more tedious than it is fun IMHO.

I read that Charisma has something to do with it, but I don't know precisely. I like to play with high CHR, from a RP perspective (I do that in every RPG game as a habit)

Hm...didn't know about that. I also like playing with high charisma. That may be why I didn't have much trouble.

Charisma is rep and store prices but its the party leader that counts i.e, the character in F1 slot so your sister is a solid leader

taking the invoker and the plot around what happens to her is the good cannon but as pointed out already, any character will do the job in the early game

p.s, my issue with Edwin is that he is a Conjurer so can't cast Identify which as i said is really one of the main roll at low levels

I usually try to have at least two mages in the group.

My last group was IIRC Protagonist(Paladin), Minsc, Jaheira, Aerie, Imoen, Jan Jansen.

Aerie, Imoen and Jan Jansen would improve haste the three fighters, all of which at this point had been specialized into dual weapon style.