I know that there have been numerous cases where a princess married a much older man who not only was old enough to be her father, but himself was a father -- and if you will, of an eligible son from a previous marriage who might have married the said princess. But instead of becoming his wife, she became his stepmother. Look at the family of King Felipe II of Spain (memorialized in Friedrich Schiller's play, DON CARLOS) and King Willem III of the Netherlands (old enough to be a grandfather to his second wife, Princess Emma of Waldeck and Pyrmont).
Like I said, some time ago, when it comes to European royal relationships, one has to be extremely precise: it's not enough, for instance, to simply say even NEPHEW. You have to specify "brother's son" or "sister's son". When it comes to cousins, in-laws, etc., the web of entanglements gets even more intricate.
I still have issues, for instance, with calling the husbands of sisters as "brothers-in-law", since I'm a stickler for strict definitions (a brother-in-law is either the husband of one's sister or the brother of one's spouse). At most, they are CO-brothers-in-law. So under the scenario you outlined, about the hypothetical double marriages between the Habsburg and Bourbon families, I'm not sure if King Louis V and his grandson would really have qualified as brothers-in-law. But then, definitions are subject to context and culture.
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