I have friends and cousins that I care for and think of as siblings. You cannot appeal to this kind of subjective things to try to sneak 15 points towards getting an ITA.
There is really nothing to be confused about, you are not entitled to these points.
IRCC's definition of sibling-->
Sibling (brother or sister)
For Express Entry, a sibling means your brother or sister related to you by:
- blood (biological):
- brother or sister
- half-brother or sister
- adoption:
- adopted brother or sister
- marriage:
- includes a step-brother or step-sister who is not related to you by blood or adoption and your parents are married or in a common law relationship
- this is true on both the date you submit your profile and when you submit your application for permanent residence
To get points for having at least one sibling in Canada, they
- must be 18 years old
- must be a citizen or permanent resident of Canada
- can be the brother or sister of your spouse or common-law partner who will come with you to Canada.
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Your situation is very simple and straight forward and there is no way you'll get those points., because you simply don't have a sibling meeting those conditions. Having someone who shares your last name or whom you love as a sibling won't make it.
You can apply sure, but your application will 100% be refused and you will have just wasted some money, time and hopes.
What you should do is just decline the ITA and wait for another ITA with your actual score. If not you can always work to improve your score or explore different alternatives.