I dont remember exactly where i read this theory so take it with a grain of salt. But according to what ive read on a thread was-

( you have to be a lord with a high right to rule with lots of fiefs and in order for marriage you have to make an alliance with her faction on top of all her vassals liking you as well.)

cant marry her if you are her vassal i think ?

I haven't tested this myself but plan to using cheats lol.

Best way i can think of is conquering most of essos under your banner and then trying the above method.