Some people think if you've seen one mustard plant, you've seen them all, but that isn't true. If plants could talk, they would tell you that they are able to recognize their siblings, or seeds that have grown from the same mother plant. Additionally, the plants will show preferential treatment toward its kin.
Seedlings from the same plant recognize each other through chemical signals in their root systems. The related plants will share resources and avoid competition in their pursuit of growth. They do this by developing shallower root systems; a shallower roots means there's more room for everybody, as opposed to using up as many resources as possible in order to knock out the plant's competitors.
Scientists believe this response may have developed as a way of strengthening the species: the more plants that perform well, the better chance for the species to survive and thrive.