These posts got me wondering, so I did a search of 'unsuitable' in the six novels. Leaving out this one (since we don't want any spoilers), I found these references to 'unsuitable matches':

====Pride and Prejudice: ====
It was a long time before she became at all reconciled to the idea of so unsuitable a match. [reference to Mr Collins & Charlotte]

But she had never felt so strongly as now the disadvantages which must attend the children of so unsuitable a marriage... [reference to Mr. & Mrs. Bennet]

====Sense and Sensibility: ====
In his marrying such a woman, therefore, there would be nothing unsuitable. [Marianne on the idea of Col. Brandon marrying an older spinster in a marriage of convenience]