It sounds strange but it's true. A study of over 3,000 mustard seedlings showed that the budding plants were able to recognize their 'sibling' seedlings from others. The way they did this is because seedlings that come from the same parent plant have a unique chemical makeup that lets the family recognize each other.
The behavioral change is dramatic. When the plants were around non-relatives, they were in fiercer competition to extend their roots and gobble up as much water and nutrients as they could, leaving out the other plants. When they were next to their siblings, they tried to form shallower roots and had more intertwined leaves.
The behavior is not exclusive to mustard plants. It was also observed in the sea rocket, a common plant that grows near seashores. They tested the theory growing a plant called Arabidopsis thaliana, in two different settings. One had chemicals from sibling plants, whereas others had unrelated plants. Their findings concluded the same results as the one for the mustard seeds.