Hi, thanks so much for the comment and feedback! (And thanks for spotting those typos, I'll fix them.) Glad to hear you enjoyed the read!
Sibling Combo: When one Brawler takes the talent, that Brawler AND their sibling have access to the sibling combo. As long as one sibling gives one of the inputs, the other sibling can use the combo when giving the remaining input, regardless of whether they're the sibling with the Talent. (If two Brawlers take this Talent, they can choose each other and they'll each have access to two different sibling combos.)
Park: You are right that double 10s (the only way to get a result of 20) is a CRIT!!, so "15-20" is wonky. The intention is to allow a Brawler, in the unlikely event of rolling double 10s, to be able choose the second result instead of the third result if they want a Flexible Item instead of a Special Item. If I were to rewrite this rule, I would have higher results stipulate that Brawlers can choose a lower result, if they'd rather gain something that's less lucrative in general but more specifically suits their current preferences.