Is the Nikah of a girl invalid if she marries without her father's permission to avoid committing a sin, even though her father did not approve of her marrying a pious non-Syed boy?
Bismillah,
Asalamu Alaykom,
If her father prevented her from marrying a suitable believing man for no valid reason, his authority drops and she may marry without his permission. The father rejecting the boy for not 'being a sayed' isn't a valid reason. This practice is usually found in the indo-pak region and is due to Hindu caste system influence. All scholars permit marriage between sayed and non Sayeds.
May Allah grant you success