You're in luck, Takumi and Elise are a thing. From what it looks like, their supports is basically a culture-exchange thing, which is actually suprisingly cute for a kind-of asshole brother and the most adorable little sister ever.
There's one for Sakura and Elise too, going up to A. Basically, Sakura is curious about Elise's violin, Sakura shows Elise a harp (maybe?), and they play together in a duet.
Found one for Elise and Ryoma as well, which is... kind of weird. It looks like he saves her from being stuck in a tree?
It looks like the families only have S-supports with each other as well as an A-support with their opposing family member. Haven't found a Hinoka / Elise support, but there's one for Hinoka / Camilla, Leon / Takumi, and Ryoma / Marx.
[spoiler=Spoiler for Hoshido ending] Since Marx and Elise both die in Hoshido, and presumably the same will happen for Ryoma and Sakura in Nohr, I think it's safe to say intermarrying the royal families is third path only.
This game is going to give me hair colour nightmares.