When you started renting out the property and it was no longer your principal residence, you had a “deemed disposition”. Then you had another deemed disposition when you gave the property to your siblings. Even though you didn’t actually sell anything, for tax purposes it's treated like you sold your home, bought it back immediately as a rental, and then sold it to your siblings. So, the same forms are used even though no money actually changed hands.
This Canada Revenue Agency (CRA) page has more information on reporting the sale (or deemed disposition) of your principal residence: Reporting the sale of your principal residence for individuals (other than trusts)
This TurboTax FAQ has more information on entering capital gains: Where do I report capital gains or losses in TurboTax Online?
