Two daughters of prince Maximilian of Saxony and his first wife Carolina of Parma married two generations of the Tuscan branch of the Habsburg-Lorraine dynasty.
first their younger daughter Maria Anna married the future Grand Duke Leopold II in 1817.
Her older sister Maria Ferdinanda accompanied the young bride to Florence and there the father of the groom (27 years older than her) fell for the elder sister and married her in 1821.
Sounds like a soap opera scenario ... while we're on the subject, has any father-son duo in royal history been brothers-in-law?
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.
When the negotiations between Austria and France over the marriage of the dauphin Louis with archduchess Maria Antonia were being finalised another marriage was proposed as well. That of the Dauphin's grandfather King Louis XV to the Archduchess' older sister Maria Elizabeth. Unfortunately she got ill and though she recovered the illness left her scarred and Louis lost interest.
Had that marriage happened two sisters would have married grandfather and grandson.
Now i know sisters marrying two brothers happened regularly, two siblings marrying two siblings from another country happened regularly.
Two sisters marrying their maternal uncles, who were also their father's first cousins happened in the Spanish Bourbon dynasty while the Portuguese Royal House saw two sisters marry a father and son, the father being the uncle of the two sisters where the son was the paternal first cousin and maternal nephew of his wife.
Im can't think of a grandfather and grandson becoming brothers in law. Louis XV and Louis XVI nearly were and i think having her big sister at Versailles to be it's leading first lady for the first few years of her marriage might have done a world of good for Marie Antoinette. Had Louis XV married Marie Elizabeth they even might have had a little Duke of Anjou.