Thank you for the fast answer.
I saw that quote in "Part 6 Information about your children" and that was clear. I was just confused because the instructions for part 8 say:
"For Part 8., Item Number 62., the following individuals are members of your household and should be includes in
your household size:
• You;
• Your spouse, if physically residing with you;
• Your parents, if physically residing with you;
• Your unmarried siblings under 21 years of age, if physically residing with you;
• Your children as defined in INA 101(b)(1), if physically residing with you;
• Any other individuals (including a spouse or child not physically residing with you) who are listed as dependents
on your federal income tax return; and
• Any other individuals who list you as a dependent on their federal income tax return."
That confused me because there I saw no stepchildren, but I will count them.
Anyway now I know what to do.