What does the term mother-in-law in bowling mean?

Answer 1

The name for a pin standing directly behind another pin, making it hard to see, e.g. 8 behind 2, 5 behind 1 or 9 behind 3. 

Answer 2

Usually it is what the pin the back row is called when another pin is directly in front of it. Sometimes also called a sleeper pin.

Answer 3

10 pin