You have no choice. Would assume your sister and her child are aware that they need to meet residency requirements to be eligible for healthcare if they don’t they can be asked to pay back any care they both received. https://www.alberta.ca/ahcip-eligibility
Proof of relationship to a relative in Canada, status of a relative in Canada and residence status of a relative in Canada (including a brother or sister for the purpose of obtaining additional CRS points) Purpose
To validate a claim of, and award points associated with, having
a relative in Canada who is a Canadian citizen or permanent resident (for the Federal Skilled Worker Program adaptability points);
an adult brother or sister who is a Canadian citizen or permanent resident residing in Canada (for additional CRS points).
Document requirements
This documentation is required if the applicant indicates that they, or their accompanying spouse or common-law partner, have a family member in Canada who is a permanent resident or a Canadian citizen.
A copy of both sides of the family member's Canadian citizenship document, Canadian birth certificate and/or permanent resident card must be provided.
Evidence that the family member currently resides in Canada must be provided (a residential lease, mortgage documents, utility bills, etc.).
The documents must show their address in Canada and should be recent (dated within six months prior to submission). [emphasis mine]
if you can provide a proof of your sibling's residence in canada that is less than 6 months old, you can claim the 15 crs points irrespective of whether your sibling is temporarily somewhere else at the moment.
Pointing out that your sister and her child are unlikely meeting the terms of receiving healthcare. You should inform them of this because it can have repercussions like possibly having to repay the healthcare they received.
You still can’t claim the points. She isn’t going on a short vacation.
if you can provide a proof of your sibling's residence in canada that is less than 6 months old, you can claim the 15 crs points irrespective of whether your sibling is temporarily somewhere else at the moment.