Generally once you light a certain number you don’t decrease that number.
Also a person can accept upon themselves to light additional candles as well for various reasons, so if you want to light for your step children there probably is no problem doing so (obviously check with your Posek). I know the wife of a Chashuv posek who adds candles for each grandchild as Hodaah to Hashem, I don’t see why this would be different. (But I’m not so familiar with the halachos — just speaking from observation)