Sister and Boyfriend Logic Puzzle - The Answer

(Solution Hidden)

Ask one of them (for example, your sister) a question with an obvious answer, such as "Is your name Eve?"

Whatever her answer, ask her the same question again.

If she is having a good day, both answers will be the truth - so her next answer will also be truthful.

If it's a bad day, both answers will be false, so you will know that the next answer will also be a lie.

Otherwise, one answer will be true and one will be false. Since, she will alternate between truth telling and lying, the next answer will be like the first.

In any case, you now know how she will answer the third question. So, the third question should be whether she is dating your boyfriend.