Can you please help me find a science fiction book from the 60s or 70s, possibly written for teens or young adults?

I've been looking for a book for the past twenty years of which I have vivid, but very minor, memories. I've been to hundreds of bookstores all over the country, but can't find it anywhere. No leads either.

From what I remember, the book was written in the 60s or 70s, and it had a large hard cover, with lots of small text. The story was about a brother and sister, raised by an angry (but unseen) mother or aunt, and the siblings escaped a somewhat gloomy childhood by entering into a fantasy world. There were large colored pictures, and the two I remember most vividly was of the brother and sister racing along a storm drain, and pretending to fly toy airplanes. The other picture I remember was of them dancing in a living room, and they had on bell bottom pants.

My brother tells me that the book was aimed at young adults, and he remembers a long, very detailed description of creating a time travel machine.

Can anyone help me find this book? It would mean a lot to me.