renown plays a major role, also NO CONTROVERY allowed, trust me, thats the killer in this...
keep licking her brothers boots, get rid of any controversy, raise ur renown to 600+ (if her familly is big, try with 700-800 renown)
he will have to accept, also make sure she's not already assigned to be married to another lord, if she is, then theres nothing u can do, but elope, wich won't help at all