one, the hair. i mean look at them both. its a mess.
two, their quirks. kota has water powers and water is usually known as blue. dabi fire is blue, and if you say "but its fire" he could have gotten it from his grandparents. kota sprays water but it could still hurt his eye or he could still get wet. so its the same for dabi, he just gets burnt.
three, again about the quirks. does anyonein todorki's family get hurt by their own family?? i mean look at them all. do they have any scars or cuts?? other than todoroki, and thats because of a kettle of boiling water.
you now have my reasoning :3