If you don't know about it, why does it matter what someone close to you does when not around you, assuming you never find out?
Maybe they like to eat McDonalds, but they are vegetarian around you. Maybe they like to dance the cha-cha, but never dance with you. Maybe they are hard-nosed and competitive business operatives but are nothing but kind and giving around you. Maybe they commit fraud, but are honest and forthright around you. Maybe they have a second family but are who they always seem to be around you.
If you can’t tell the difference, then what is the difference to you? Do you want to know the truth, even if it means it will really change the way you see this person, and may even end the relationship? Or is the relationship the most important thing, even if you don’t know the whole of the person?