You mentioned fosterage works just as well as marriage. Would it make sense for a person who was fostered with another family to later marry someone from that family? Or would that be considered wasteful, since a bond was already established?

It’s not unknown, a sort of belt-and-braces approach to building an alliance that builds on the idea that such a marriage might be more successful if the two betrothed actually knew and hopefully liked one another rather than being total strangers.