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.
If someone has a cast iron stomach, they have a strong stomach that can digest unusual food and even bad food without being ill.
Jimmy must have a cast-iron stomach, when he drank milk with lime juice and felt perfectly fine afterward.