In chapter 13 we have:

"Oh! my dear," cried his wife, "I cannot bear to hear that mentioned. Pray do not talk of that odious man. I do think it is the hardest thing in the world, that your estate should be entailed away from your own children; and I am sure, if I had been you, I should have tried long ago to do something or other about it."

Jane and Elizabeth attempted to explain to her the nature of an entail. They had often attempted it before, but it was a subject on which Mrs. Bennet was beyond the reach of reason, and she continued to rail bitterly against the cruelty of settling an estate away from a family of five daughters, in favour of a man whom nobody cared anything about.

The wording at the beginning of the second paragraph above struck me as odd. Surely Mrs. Bennet understands what an entail is, so Jane and Elizabeth can't be explaining the law to her. So, what are (and what have they been) trying to explain?

Does the narrator mean the girls were trying to justify it to her and defend it in a moral sense? That seems unlikely. Were they trying to deflect blame away from Mr. Collins, arguing that it's not his fault the girls might lose the house when Mr. Bennet dies? That seems most likely, but still not quite satisfying.