(No Spoilers) Is Marriage the only way for Houses to secure alliances with each other?

Like what would happen in a scenario where two Houses wanted to make alliances with each other but they only have Sons or Daughters between them, how would that go then? can you secure alliances via Fosterage? or via Hostages, kinda like Theon to the Starks? or do they make some sort of compromise to come into an agreement, or, they just stop and come look for other houses for potential marriages, I'm sure contracts are not a thing in Westeros since binding a will to a piece of paper is surely not gonna end well, looking at you Robert.