The reader is just now aware of it.

It is interesting that Jane Austen introduces the disconnect between Mrs. Bennet and Mr. Bennet at the end of Ch. 1, but does outright come and say that Mr. Bennet has no respect for his partner until the beginning of Ch. 42:
Had Elizabeth's opinion been all drawn from her own family, she could not have formed a very pleasing picture of conjugal felicity or domestic comfort. Her father, captivated by youth and beauty, and that appearance of good-humour which youth and beauty generally give, had married a woman whose weak understanding and illiberal mind had very early in their marriage put an end to all real affection for her. Respect, esteem, and confidence had vanished for ever; and all his views of domestic happiness were overthrown.

It is our knowledge (and Lizzy feels it, but no one else in the family has) that informs us of Mr. Bennet's impropriety:
But she had never felt so strongly as now the disadvantages which must attend the children of so unsuitable a marriage, nor ever been so fully aware of the evils arising from so ill-judged a direction of talents; talents which, rightly used, might at least have preserved the respectability of his daughters, even if incapable of enlarging the mind of his wife.