I have read a ruling from Seyed Sistani saying that if a father keep refusing to marry his daughter with suitable candidates without a valid reason, then he loose his wilayah over his daughter in the question of marriage. Is this statement true?

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Yes that’s true. If her father keeps refusing those candidates who are suitable for her (both religiously and socially—meaning they are religious, and socially they are deemed appropriate for her), then she no longer needs his permission to marry someone who is religiously and socially suitable for her.

However, she should be careful not to do something that would backfire or make her father take a very strong stance against her, as that could lead to long term tension.