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.