What did Jon Arryn gain out of Southron Ambitions (pre-Rebellion)? It’s not like he, Elbert, or Denys was betrothed/married to anyone in the bloc.

goodqueenaly:

Thanks for the question, Anon.

Well, for one, we don’t know what marital arrangements Jon Arryn was hoping to make for nephew Elbert (presuming that Elbert was not married when he died). There’s no mention of a potential Lysa-Elbert match, though surely the thought must have passed Arryn’s mind at least once (even if ambitious Hoster wanted richer Jaime Lannister for his younger daughter). If indeed the conspirators saw the advantage of bringing in Tywin, then Jon Arryn might have thought to win Cersei as a bride for Elbert. Certainly as well, even if Elbert married someone of lesser distinction in this generation, his children might well be wed to the children of Brandon and Catelyn, or Robert and Lyanna.

But even without specific marital arrangements, Jon Arryn benefited from inclusion in the SA pact. For one, if the purpose of the pact was to protect the Lords Paramount against the whims of the Iron Throne, then the Vale gained the assurance that Riverrun, Winterfell, and Storm’s End would protest on its behalf. For another, Jon Arryn was put in a position of key influence over Ned Stark and Robert Baratheon: fostering them from an early age, Jon Arryn had the ability to shape their views and mindsets, to mold this next generation into worthy heirs of the senior one. He would give them their political education, and in turn they would remember him as not only a beloved father figure, but a crucial adviser and shrewd elder statesman. There are worse fates than grooming the future Lord of Winterfell’s natural first adviser and the future Lord of Storm’s End and close heir to the throne.

The Queen Regent (NFriel)

Seconded. Quick addition:

Keep in mind, fostering isn’t just hosting someone. Jon Arryn became the foster father of Ned Stark and Robert Baratheon, and they became his foster sons. Westerosi treat foster relationships as equivalent to blood relations, hence why Theon is called a kinslayer for “killing” his foster brothers Bran and Rickon. 

So it’s just as close an alliance as any marriage.