False reports to CPS happen all the time. Unfortunately for you in this case - your local CPS agency must investigate to make sure there are no children in harms way. Your best move is to make yourself available ASAP so the investigator can see that you do not have a drug problem.
Having a social worker speak with your child ALONE is one of the scariest things for any parent. You can insist that a school representative be present when the worker speaks with your child. You could also insist they interview your child at home to avoid any embarrassment for your child at school.
It may be true that they do not have a case - well of course they don't have a case if you and your sister do not use drugs - but refusing to cooperate with the investigation in itself sometimes creates a case! In my opinion, you should cooperate.
Regarding the warrant: It seems that some judges give out warrants to CPS like candy - so insisting on one only prolongs the process. You can insist on one - they need one to come in your home. As I tell my clients - do you want to be the one who pays some attorney to take on CPS and go on up to some higher court with your case while they hold on to your children?
Keep good records of who you speak with - dates; name; phone number; position in the agency; what they tell you. Don't sign anything they refuse to give you a copy of.
If you can afford it - consult with a local Juvenile Dependency attorney - they may be able to be present at your interview with the worker - but not likely at the child's interview.