] No, not breaking the entail, but continuing it. Mr Bennet's estate was entailed on the male line. His son would have inherited it after his death, and would then, he presumes, looked after the female Bennets.

In Chapter 50 (after they have received Mr Gardiner's letter about Lydia & Wickham):

When first Mr. Bennet had married, economy was held to be perfectly useless; for, of course, they were to have a son. This son was to join in cutting off the entail, as soon as he should be of age, and the widow and younger children would by that means be provided for. [emphasis mine]

I assumed that cutting off the entail would ensure that Mrs Bennet and the girls would be provided for even if they outlived this theoretical son.