The question you opened
Four sisters and their brother
A family has 4 sisters, and each sister has exactly one brother.
How many children are in the family?
Five
The four sisters all share the same brother, so the family has four girls and one boy. Five children in total.
A family has four sisters, and each sister has exactly one brother. There are five children in the family. The wrong answer of eight comes from multiplying: four sisters, one brother each, four plus four. It treats each brother as a separate person, and that is the whole trick. All four sisters are talking about the same boy. From his side, he has four sisters. From each sister's side, she has one brother.
Same family, five children, described from different chairs. Drawing it settles the matter instantly. Put four circles for the sisters and one square for the brother. Now check the clue: does each sister have exactly one brother. Yes, the same square, four times over. The picture holds. The idea underneath is that relationships get counted from both ends. If you count friendships by asking each person how many friends they have and adding it up, you will count every friendship twice.
If you count handshakes by asking each person how many hands they shook, same problem. Anything shared between people gets counted once per person unless you are careful. You can push this puzzle further. Ask how many sisters the brother has. Four, obviously. Then ask how many sisters each sister has. Three, because she does not count herself. That last one catches almost everybody, and it is the same lesson from another angle: who is included in the count.
A harder version to try on someone. A family has three brothers, and each brother has two sisters. How many children are there. The answer is five again, three boys and two girls, and the same shared counting is doing the work. The habit worth building is to ask one question before adding anything up: is this thing shared. If it is, adding once per person will overcount it, and the picture on paper will show you by how much.
Technique: Ask whether the count is shared