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.
To do domething based on your emotions rather than for rational reasons
I can't believe that Tom beat his classmate. He's always the one who let his heart rule his head.