"Dearest Marta, hi" Is it correct to begin an informal letter using "Dearest"?
Yes! Since it's an informal letter, you are free to begin it however you wish.
All these are fine. Just remember that the salutation you choose sets the tone for the rest of the letter. Dearest is often used to refer to someone who is very close to you, like a lover or your best friend, so don't use it unless you really like Martha a lot.
Also, I wouldn't start a letter with "Dearest Martha, hi" like in your example. Your salutation is the greeting that starts your letter, and it takes the form
[Introductory word] [Name],
Since you already introduced your letter with Dearest Martha, there is no need to use hi. After your salutation, start a new line and begin the body of your letter.
Help this helps!