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".

I think "small" is also correct for "wadogo"

Do you say kaka and dada for older siblings as well?

When theres no ni/si before the wadogo it means younger not small

Could this also mean "Brother and younger sisters"? How do we know that the adjective refers to both subjects?