The Penderwicks on Gardham Street by Jeanne Birdsall
Yes, those wacky Penderwick sisters are back again, plotting, scheming, secret meetings, only this time they are working on the "Save Daddy Plan". They figure the way for them to keep from getting a stepmother is to plan some really bad dates for their dad.
They are not above enlisting the neighbors to help them get the most terrible people they can come up with. Finally their Dad gets fed up with the whole dating process, makes up a new girlfriend "Marianne" who he spends all his extra time with. This is very frustrating for the Penderwick sisters and they set out to find, and meet "Marianne".
As things get more and more muddled, the Penderwicks are plotting a subterfuge that only these sisters could come up with. But wait who is Bug Man? Is he important? Will the sisters ever get a new mom? You should really read the book!