OTL, Madame de Montespan was betrothed to Louis de la Trémoïlle, later duc de Noirmoutier. Unfortunately, Louis went and got involved in a duel and had to quietly leave the country, and she married the marquis de Pardillan instead.
Now, Louis Alexandre de la Trémoïlle is a member of one of the families of princes étrangers at the French court. At the time of becoming mistress to the king, OTL, Athénaïs would be a widow (Louis Alexandre died in 1667), with two children (she was extraordinarily fertile OTL, so I don't see why that would change here). Would she still become the maitresse-en-titre? Or would Louis be less interested in a widow, and pass her by completely?