I'm not going to tell my aunt that her husband has a second family in another country
Why are you not going to tell her?
I don't want to hurt her feelings.
I'm sorry that you had to know that, since it is disgusting to lead two lives like this. I'm also glad you're not going to share it, because your aunt doesn't deserve any following pain and anguish the truth will cause.
Yeah I can't imaging how she would feel.
Even thought it is a bad situation it is better to stay out of other peoples business. It might be useful to let him know that you know about it and don't want to have to lie.