Good point about Bronze Yohn and Ned being war buddies. But would marriages 3 or more generations back really count for much in a feudal aristocracy? I mean with noblity intermarrying, you go far enough back, you’ll probably find that almost all houses of closeby regions are related. But does not seem to have stopped wars or feuds.

The thing about feudal aristocracy is that they take issues of blood relation really seriously, because it’s linked to claims of inheritance, and the key poltiical units are extended families rather than nuclear families. So you back your relatives’ claim because it might become your claim down the line, or because them being in power means you can benefit (through offices, perks, fosterages, marriages, etc.), but ultimately because it’s good for the Family Business. 

Now, it’s not a guarantee – the fact that Richard Plantagenet, the Duke of York was married to Cecily Neville, the aunt of Richard Neville the Earl of Warwick, certainly helped the formation of a political alliance between the two Richards during the Wars of the Roses; on the other hand, Richard was also a cousin of the Percys through his mother (as well as through his wife, whose sister had married into the Percys), but they sided with the Lancastrians. 

I’d describe it as more of a tendency than an absolute.