I just looked up when you use "si" in French to mean "yes". It is in response to a negative question. For example, "You're not hungry are you?" The response would be, "si" meaning: yes, I am hungry. So the way Duolingo used it here would not be best usage. Since the yes translation is in isolation, it should be "oui" - in my opinion.
Oui = the common way of saying "yes", in answer to a question
Ouais = an informal form of oui
Mouais = a variant of ouais, they're equivalent
Si = used in reply to a negative question, like "you don't like coffee, do you?"
Vous aimez le café ?
Tu aimes le café ?
Tu n'aimes pas le café, non ?
I don't know what the sentence about "big brain" means. I know I haven't got one :) But I do know that "Si" is used in French for "yes" when answering a question with a negative, like "no" or "not" in it, like "He does not work, does he?" and "You have no idea, have you?"