The major distinction is: “In” is used when you just name the group.” You do that correctly in your third sentence. “Among” is used when you refer to the group members, so your fourth sentence should read: “He’s the smartest among the people in the family.” As far as the first two sentences: “Order” is a bit of a confusing word here. I think you mean “birth order”: among the children in your family: were you born first, second, etc. That having been said, even if we put in “birth order”, your sentences are not natural. If you wanted to know the whole birth order, we would probably ask it like this, if it were all in one sentence: “what’s the birth order among you and your siblings?” Or “what’s the birth order in your family?” If you wanted to know where the person you are talking to is in the birth order, you’d most naturally ask it like this, indirectly: “Are you the oldest child?” The person would then respond with something like one of these: “Yes.” “No, I’m second.” “No, I’m the youngest.”

1+ 2) What is your position in your family? (in this case, "order" makes no sense) 3+4) He is the smartest in your family Lucia is correct as her offering is in clear English but no one would actually write that clumsy sentence when the simpler one exists.

Thank you

Hi, Kai! 2 and 4 are incorrect, since "among" is a preposition that is typically followed by a plural noun phrase , hence you could say "He is the smartest among his family members".