Student or Learner
This sentence is correct Or not "This letter is from a uncle of mine
Note my corrections above, particularly the correct way to formulate a question ("Is this ...?" not "This is ...?") and the correction to your indefinite article before a vowel sound.
Your sentence is grammatically acceptable but a little unnatural. We would say "This letter is from my uncle" or "This letter is from one of my uncles".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.