Your immediate family includes your father, mother and siblings. If you have 1 brother and 2 sisters, then you have 3 siblings. you and your siblings are the next generation, and your children and their cousins are another generation.
If there is no Will, the deceased's next of kin can apply for probate (for example. their brothers and sisters (or their children if they have already died).. If your father had no surviving spouse or civil partner, then the estate