Found: YASF. Teenage brother & sister and alien brother & sister save earth. Read 1959-1962

Some aliens want to blowup earth to release a gas that the aliens use for power. Earth brother & sister team with alien brother & sister to stop the destruction of earth. Teenagers discover that one of their teachers is working with the aliens to help blowup earth.
Book was in my junior high library for period of 1959 to 1962. I think it was new.
Cover showed a flying car-like vehicle with the 2 teenage couples in it.

>1 nssmith2024: It doesn't line up exactly (two brothers instead of a brother and sister), but perhaps The Mystery of Marr's Hill?

>2 slimikin: slimikin: Thanks for your reply but that is not the book that I am looking for.

>3 nssmith2024: Ah, that's disappointing. After reading this Kirkus review of the book, I thought it was a perfect match for your description. I hope someone else has better luck tracking this down!

nssmith2024 messaged me and told me The Mystery of Marr's Hill is the book. They also pass along their thanks to slimikin.