Imagine Sally has 1 brother (Mark) and 1 sister - Mark has two sisters (Sally and 1 other) and no brothers - not right

Imagine Sally has 2 brothers (Mark and 1 other) and 2 sisters - Mark now has 3 sisters (Sally and 2 others) and 1 brother - not right

Imagine Sally has 3 brothers (Mark and 2 others) and 3 sisters - Mark now has 4 sisters (Sally and 3 others) and 2 brothers - correct