The two words are very similar. They are often used interchangeably.
The word PERTAIN means to belong to or be a natural part of.
For example, chickenpox is an illness that PERTAINS to childhood.
Chickenpox is a common, usually mild childhood disease.
The word RELATE means connected as with family relationships.
For example," chicken pox is RELATED to shingles, both are caused by the same virus".
These two diseases are connected.
Hope this helps.