Why is it so hard for me to explain how to ride a bike?

This is because you can have very good nondeclarative memory and yet very poor declarative memory for the same thing!

If you know how to ride a bike – you can hop on and, without training wheels, pedal down the street – then you have developed strong nondeclarative memory for the motor skills of riding a bike.  But that is different than having a declarative memory for riding a bike - an ability to describe what it is that you are doing. 

When you were first learning how to ride a bike, you might have had this ability to describe what you were doing - you might even have been thinking to yourself about what you needed to be doing at each step (“Now I put my foot on the top pedal…Now I shift my weight…”). But after practicing the skill, you no longer needed to think about each step, and in fact it probably became harder and harder to verbalize each step. 

This transition from using declarative memory to using nondeclarative memory as we carry out a skill is one of the things that indicates we have acquired expertise in the skill.  So, while it might be frustrating that you can’t help your younger sibling by explaining how to ride a bike, the fact that it’s hard for you is evidence of how hard you’ve worked to learn that bike-riding skill well!