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.
Every sinner gets one sin for sinning as we read in Quran: (Whoever brings a good deed shall get ten times like it, and whoever brings a bad deed shall have only like it)
(Sura 6, verse 160)
The wives of the Prophet (SAWA) were been warned that if one of them does bad, she will get double the punishment. (Sura 33, verse 30). That is because of the great responsibility on every wife of the Prophet (SAWA) to obey Allah and the Prophet and be a good example for others.
Descendants from Ahlul Bayt (AS) are not included in this rule. This is according to the verdicts of most of our Maraaje' of Taqleed.
This claim is not true. Great Sadat scholars including Maraaje' of Taqleed, have allowed marriage of their daughters with non Sayyed Momins. The claim mentioned in your question came from certain culture with no concrete evidence in religion.
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