So to sum it up literally nothing in those pages gives any actual hint for the particular reasons for their marriages. If you have to make huge assumptions then there really is no point to this.
You're welcome to come up with more plausible in-universe reasons for the phenomenom to still be kicking around than good old dynastic alliance-building.
I'll wait.
And you forget that this story takes place in the future and even in modern Japan using marriages to combine families is seen as archaic. Modern day arranged marriages are almost entirely done for purely business reasons or if the the participants are getting too old to waste time dating to find a suitable partner. Now this takes place in the far future, I doubt that they regressed into inforcing feudal traditions. Even if it was about combing two families, why do these two families absolutely require a combination that they are willing to sell their daughters for? Its these important details that I need or else Im just going to roll my eyes every time the subject is brought up.
You said it yourself - "business reasons." AKA what I've been repeating ad nauseum about strategic alliances, power and wealth.
You seem to have some rather odd, if not downright naive, ideas about how powerful upper-class clans have universally operated everywhere and still do in many parts, and why.