Can a Sayyed girl marry a Sunni boy? What is the ruling in Islam about this matter? If it is allowed, but the parents do not agree, then what can be done?
Bismihi ta'ala
A Sayyid male/female is like every other Muslim, nad can marry a non-Sayyid/non-Sayyidah, Shi'ah or Sunni, with no difference. The one condition is she has assurance that she will not leave the Ahlul Bayt School of Thought, or negatively influenced or forced to become Sunni.
As for parents disapproval, there must be correct understanding between families, and one must always try to get the consent and blessings of parents.
And Allah knows best.