In chapter 18 we read:

She (Elizabeth) looked at her father to entreat his interference, lest Mary should be singing all night. He took the hint, and when Mary had finished her second song, said aloud, "That will do extremely well, child. You have delighted us long enough. Let the other young ladies have time to exhibit."

Mary, though pretending not to hear, was somewhat disconcerted; and Elizabeth, sorry for her, and sorry for her father's speech, was afraid her anxiety had done no good.

I appears that Elizabeth wanted her father to interfere but was sorry for the way that he did it. I don't understand in what manner his behavior was faulty but I think JA was trying to convey that it was. Of course, I may be influenced by P&P2 in this instance as this is the scene Lizzy flashes back to when she's imagining her family's bad behavior while reading the letter.