While posing for a family photo, the father is standing to the right of the son and left of the grandmother. The mother is sitting to the left of her daughter and right of the grandmother. Who is seated in the middle?

Let us arrange the family members from left to right:"the father is standing to the right of the son and left of the grandmother" $⟹\text{Son}-\text{Father}-\text{Grandmother}$ "The mother is sitting to the left of her daughter and right of the grandmother" $⟹\text{Grandmother}-\text{Mother}-\text{Daughter}$ Combining both sequences together:
$$\text{Son}-\text{Father}-\text{Grandmother}-\text{Mother}-\text{Daughter}$$
There are 5 family members in total. The one sitting exactly in the middle (3rd position) is the Grandmother.