Is the sum from the sale of a deceased father's car considered an inheritance, and do the children of a deceased sister fall under the heirs in this case?

The children of the deceased sister do not inherit from the grandfather because they are considered uterine kin (dhawu al-arham), and the deceased sister does not inherit from her father because she died before him. Regarding the car for which you paid a portion of the price: if it was a donation to your father, then it entirely belongs to the heirs. If you paid with the intention of partnership and the heirs confirm your statement, then you are a partner to the extent of what you paid. If the heirs deny your claim, then the judiciary is the arbiter. It is necessary to refer to the Sharia courts or people of knowledge to inventory the heirs and divide the inheritance, as there may be wills or debts that take precedence over the rights of the heirs.