If you are the sponsor, you don't need to fill out the form.
You wife needs to fill out. Under Applicant she puts her name, under Spouse - your name, under parents - her parents' name, under children - all her children's names (even adopted ones and even if they are not being sponsored).
Your step daughter needs to fill it out only if she is 18 years old or more and she is a dependent of your wife.
Dependent children
Refers to the children of the applicant or those of the spouse or common-law partner.
They must:
- be under the age of 22 and not have a spouse or common-law partner, or
- depend substantially on the financial support of a parent and have been continuously enrolled and in attendance as full-time students in a post-secondary institution accredited by the relevant government authority since before the age of 22 (or since marrying or entering into a common-law relationship, if this happened before the age of 22), or
- depend substantially on the financial support of a parent since before the age of 22 and be unable to provide for themselves due to a medical condition.