]"She is so fond of Mrs. Forster," said she,

I can't imagine that Mrs. Forster likes to be Lydia's friend again, and if she does b/c of her imprudence and silliness, at least Mr. Forster would prevent such a relationship after what Lydia and Wickam did to them. But what astonished me that Mrs. Bennet still think of that! It seems that she has the ability to delete the unwanted matters from her brain and install the ones she likes, and what a brain she has!

Also her partiality to Lydia is quite obvious and it doesn't seem that any thing will ever cease it, here what she said to Mr. Gardiner:-

"And as for wedding-clothes, do not let them wait for that, but tell Lydia she shall have as much money as she chuses to buy them, after they are married."

"as much money as she chuses"!! No wonder that Wickham agreed to take her with him, for sure she might told him that her mother gives her as much money as she chuses.
What I know, it is a great pain to live with such a mother and sister