PLEASE HELP! Does anyone know of a book about a brother and a sister who go to visit thier grandmother?

It's abut a brother and sister who come from chicago and go to visit their grandmother during the depression. they stay for a summer and the grandmother is a bit eccentric. she fakes winning a blue ribbon for her gooseberry pie at the state fair so that she can get a free plane ride for her grandson because the plane can't lift her up. another time she puts a dead mouse into a milk bottle to trick the milk boy who has been blowing up mailboxes. There are other things she does but i can't remember them, there was also a sequel where the sister went to visit, but the grandson had gone off to war. i can't find this book anywhere and it's driving me crazy!