ANSWER:

This is not a valid reason to reject a proposal as there’s no caste system in Islam! Nevertheless, this is the decision of the father who is her guardian. 

Sir, I wanted to marry a girl in my Locality and the girl was also interested in marrying me . However her parents rejected the marriage proposal and the reason was that they belong to shah caste and they don’t marry their daughters outside their own caste. They say they are the descendants of Prophet Muhammad (P B U H). Is this reason justified and whats your suggestion for me as I am heartbroken