Actually, it depends on your sisters':
- Age when the APPLICATION was INITIALLY FILED
- Their marital/common-law status NOW (which will be reassessed when the visa is issued and when they land)
So, if one or more of your sisters was/were under 22 (or over 22 and substantially dependent on parents for financial support) when the visa was filed AND if she is single then, yes, they should add them to IMM-0008 application as dependents.
See sources:
"5.17. Lock-in age of principal applicants or accompanying family members
For dependent children, only the age is locked in, not dependency. If the applicant is
under the age of 22 at the time of “age lock-in,” then the fact that they surpass that
age by the time a decision is made on their application does not affect their eligibility.
The age of less than 22 years for dependent children is locked in on the day CPC-M
or CPC-V receives a complete sponsorship application and minimum requirement."
http://www.cic.gc.ca/english/resources/manuals/ip/ip02-eng.pdf
and also OP-2
"5.13. Who qualifies as a dependent child?
R2 and Section 6 in this chapter describe who qualifies as a dependent child.
In order to meet R2(b)(i) of the definition of “dependent child”, a child must be under 22 years of
age and not a spouse or common-law partner on the date when the IMM 1344AE is received by
CPC-M, and not a spouse or common-law partner when the visa is issued, and when they arrive
in Canada.
Not a spouse or common-law partner means that the dependent child must not be married or
involved in a common-law relationship. A dependent child who is single, divorced, widowed, or
whose marriage has been annulled is not a spouse. Similarly, if the dependent child was involved
in a common-law relationship but that relationship no longer exists, they may be considered to
meet the definition.
R2(b)(ii) & (iii) describe children over the age of 22 who may be considered dependent children if
they are substantially dependent on their parents for financial support. This includes full-time
students enrolled in accredited post secondary institutions or children with a physical or mental
condition. In such cases, officers must ask for documentary evidence of full time attendance at
school, evidence of the institution's accreditation with the relevant authority, evidence of the
physical or mental condition and evidence of financial dependency on parents."