They are all members of the Brassicaceae or mustard family of plants. They are also known as cruciferous vegetables because their flowers have four petals that resemble a crucifix or cross (from the Latin, cruciferae). These plants contain sulfur compounds, called glucosinolates, which when broken down (by your gut bacteria or plant enzymes) can have antioxidant, anti-inflammatory, and anti-cancer effects. Mustard greens contain about twice as many glucosinolates as kale.
Fun fact: Did you know that the spicy chemicals in mustard that make your nose burn aren’t actually created in the plant until you take a bite out of it? When you bite into or chop up the leaves of cruciferous plants the glucosinolates get converted to other spicy or bitter chemicals! You’ve released enzymes in the plant cell that allowed this reaction to happen.