Does Miss Marple have a sidekick?

Not really. Miss Marple had several friends who would acompany her sometimes or be referred to, but she didn't have a Watson or a Hastings.

There is no exact explaination as to why, but maybe this quote from Agatha Christie about Miss Marple will help:

" There was no unkindness in Miss Marple, she just did not trust people. Though she expected the worst, she often accepted people kindly in spite of what they were."

It is probale that although we all know that Miss Marple is compassionate and able to see good in people, that this inate distrust (possibly from an event in her past, maybe similar to Agatha Christie's herself) for others makes one trusty companion who would acompany her on each adventure impossible.