Can I say "Younger brother and sister". Isn't dogo plural because there are two nouns? or would that be "kaka na dada mdogo"?
The adjective takes agreement with the noun it qualifies, except for those of Arabic origin of course. If the adjective qualifies several nouns, the adjective will agree with the last noun if they are of mixed classes, or "KI-/VI-" class of they all denote things, or with each of the nouns if they are of the same class. Also, you wouldn't be able to differentiate between "brother and sister" and "brothers and sisters" if both were taking the plural adjective agreement. So, younger brother and sister is "kaka na dada mdogo", younger brothers and sisters is "kaka na dada wadogo".
Yes, we've learnt "kaka mkubwa" for older brother/big brother - kaka and dada are simply the siblings the adjectives indicate whether they are older or younger if necessary