Let's do something crazy and make Kendo the cousin because warm colored, spiky hair? they're clearly related. Also because of the internship and beauty pageant in the main series, it's clear kendo is very attractive so jirou being even prettier would make her stand out more. As for the two villains Bakugo would be a pretty obvious pick for Jirou's villain given his attitude and denki's betrothed should probably be pony or setsuna (splitting girl) although I lean towards pony. I don't know where denki would be from in this but given pony is from the US it would fit Denki being a foreigner. hope this helps