Sister and Boyfriend Puzzle

This puzzle is from the March 8, 2009 "Ask Marilyn" column in Parade Magazine:

You suspect that your sister Eve is dating your boyfriend, so you confront them.

On her good days, she always tells the truth. On her bad days, she always lies. On other days, she alternates between telling the truth and lying.

Your boyfriend is the same way, but never on the same days as your sister.

They agree to answer a total of three questions between them.

What questions should you ask to find out?

Please click here for the answer.