@Cosey what I want to know is have you called cic? and asked them if they made an error that these are your step children? I am asking because it's an error I have not heard of asking DNA when u have said they are not your children. more like the mother and kiss should conduct DNA...
I would ask you yo call CIC Mississauga and ask them did they male a mistake? have them escalate to their manager to look into if AVO MADE AN ERROR if they say it's not an error then complete the DNA kit. it's expensive so you want to make sure on their end. Have the sponsor call 1-888-242-2100 Monday to Friday 8am to 4. indicate you want to make sure they did not make an error with your package because you have said the children are not yours. please do that first because they have the manager look into the case and get back to the visa officer then they will tell you how to proceed.