Thank you
@Rob_TO for your response.
I haven’t wasted the lottery spot. I still qualify to sponsor 3 instead of 4. And my all resent past 3 years do meet requirement to sponsor 3 additional family member.
As mentioned, you are in a tricky situation.
Your app is due May 25 to IRCC, at which time the sister will still be 21 years old and a dependent. You can't exclude her from the app, she MUST be included as a non-accompanying dependent, and she MUST be counted in your family size. So you are basically submitting an ineligible app upfront.
However just a few days later she turns 22, so after this happens you can request to IRCC that you exclude her completely as a dependent in the application.
The visa officer may allow this, in which case would calculate your 3 years income for family size of 7 (since by the time they even open your app to start checking your income, she will already be over 22 so can request to exclude her).
OR, the visa officer may see at the time you submitted the app you had family size of 8, see you don't meet LICO for family size of 8 in 2015, and reject the app based on that. They may not accept the exclusion of sister from family size.
You can submit and hope for the best. It may not be until 2019 though until you get the decision on income check and sponsor approval.