Is it permissible in Islamic law to reject a marriage proposal if a girl seeks to avoid sin by marrying someone who is equal to her in all aspects but is not from the Syed lineage?
Bismihi ta'ala
There is no "sin" in rejecting a marriage proposal, and there is no "sin" in marrying a non-Sayyid.
You must marry someone who, along with having the fundamental qualities of being religious and good akhlaq, but also compatible with you as well, and whether he is a Sayyid or not is irrelevant.
And Allah knows best