In your view, what would Lady Stoneheart do if she captured Olyvar and Perwyn Frey — the Freys whom she knows for a fact were not part of the Red Wedding (and probably would have opposed if it they could have)?

Catelyn Stark knows that, I don’t think LSH does, otherwise she wouldn’t be nearly as violent towards Brienne as she is. 

Catelyn Stark is a reasonable, intelligent woman. LSH is Nemesis, and Nemesis is implacable, unrelenting, and without mercy.