Tracking down an old sci-fi book

I'm trying to track down an old sci-fi book I read when I was a teenager.

The book was at the Cottage Grove, OR Public Library around 1982-1985.

The plot was about a brother and sister who were scientists or spies, or both.

The brother once used a series of pills to change his weight, height and appearance.

Toward the end of the book, the brother takes a space ship that his sister invented, removes a governor so that he can travel faster than the speed of light and goes back in time several hours.

This is really vague, but thanks for any leads you can share.