As Sir John said, back when Brandon insisted on leaving their intended excursion to Whitwell to attend to his mysterious business in London, "There is no persuading you to change your mind, Brandon, I know of old,when once you are determined on anything." No one objected to this remark, so Mrs. Jennings probably agrees with it. If he had decided against Marianne (as far as she knows), he would not be persuaded back into it.

Also, Mrs. Jennings has first hand experience with trying to match Colonel Brandon up with Charlotte, and he was not interested and never changed his mind on that. Despite Charlotte's claims that her mother didn't think Brandon was a good enough match for her, I think Mrs. Jennings was probably very interested in making that happen, and I think that the visit she and Charlotte paid to Delaford was probably part of the plan--to no avail.