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"An unhappy alternative is before you, Elizabeth. From this day you must be a stranger to one of your parents. Your mother will never see you again if you do not marry Mr. Collins, and I will never see you again if you do."
One hand: Mr. Bennet steps up to his fatherly responsibilities when he has to. Other hand: He undermines his wife's influence. Also, think about it from Mrs. Bennet's perspective. Sure Mr. Collins is an idiot, but when Mr. Bennet dies, Mrs. Bennet's daughters will be poor and kicked out of their home. You can't blame her for wanting her daughters to have a secure future, even if it means marrying silly men like Mr. Collins.
In her own past behaviour, there was a constant source of vexation and regret; and in the unhappy defects of her family, a subject of yet heavier chagrin. They were hopeless of remedy. Her father, contented with laughing at them, would never exert himself to restrain the wild giddiness of his youngest daughters; and her mother, with manners so far from right herself, was entirely insensible of the evil. Elizabeth had frequently united with Jane in an endeavour to check the imprudence of Catherine and Lydia; but while they were supported by their mother's indulgence, what chance could there be of improvement? Catherine, weak-spirited, irritable, and completely under Lydia's guidance, had been always affronted by their advice; and Lydia, self-willed and careless, would scarcely give them a hearing. They were ignorant, idle, and vain. While there was an officer in Meryton, they would flirt with him; and while Meryton was within a walk of Longbourn, they would be going there forever. (37.17)
Elizabeth takes a cold, hard look at her family and doesn't really like what she sees. It's as if she's finally able to view her family the way Mr. Darcy or Miss Bingley might. She always knew her mom was kind of silly and unable to reign in the wild, hormone-driven young Bennet girls. The problem is, Mr. Bennet never steps up and acts like a father to his girls. He'd rather just sit back, make fun of his daughters, and let them make mistakes. It's no wonder Mr. Darcy, Miss Bingley, Mrs. Hurst, and even Mr. Bingley find the Bennet family shocking.
But she had never felt so strongly as now the disadvantages which must attend the children of so unsuitable 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. (42.3)
A bad marriage can screw up the kids if the parents fail to be watchful of their children's characters. Here Elizabeth realizes for the first time that her father isn't exactly perfect. It's just one of many revelations about other characters that Elizabeth must undergo.