Medicals are done to establish a couple of things:
1. You or your family member does not any disease that is a potential threat to any Canadian i.e. HIV+, AIDs etc.
2. You or your family member is not a potential threat to public safety i.e. active tuberclousis.
3. You or your family member does not have a disease that will put excessive demand on the Canadian Medical Systems i.e. they have to spend more money on you than an average Canadian.
Remember, if one member of the family is inadmissible on the health or security grounds, the whole family is. There is no splittling of the family.
I would suggest you is to take it easy.Don't drink too much alchohal or smoke pot or do any illegal drugs before you go for your medicals. Don't think about it too much. It is what it is and nothing will change in the next few days.
Thanks!