So far all of the siblings with major screen time are the opposites of each other. Not polar opposites mind you, more like the flip sides of the same coin.
both Komis are adored silent being. one because of a crippling inability to speak; the other just cooly saunters through life and simply has no use for words
both Tadanos are extraordinarily average. Brother Tadano has a near supernatural instincts when it comes to sensing others intentions, but worries about butting into their affairs; while sister Tadano charges in full bore and relentlessly, but completely misunderstands the those she involves herself in. (which makes her surprising intuition concerning her brothers love life even more amusing)
Finally the Katais, who are both intimidating to those around them. Ai is a legitimate "hard-liner" who is openly agitated by softness in all its forms and has a will of iron. Her brother on the other hand has the internal fortitude of a small rabbit and is in fact soooo skittish it ironically terrifies others. Even the people they end up becoming enthralled by are opposites.