Be (all) brothers/sisters under the skin phrase
Although two or more people are obviously different in some aspects such as appearances, position, etc., they have similar feelings or thoughts.
They are brothers under the skin, but they seem to not know about it.
My friend and I are sisters under skin, as we both hate spicy food.
The verb "to be" should be conjugated according to its tense.
People tend to like forbidden or illegal things just because they are forbidden or illegal.
He must have believed that stolen fruit is the sweetest, and he, therefore, cheated on his partner.