The only one who can give the go ahead is the father as he is the guardian, despite all the differences and disputes with him.
If the boy proposing is a good one yet the father still insists to refuse him and refuse anyone else, then the only alternative is to go to the Islamic judge and complain to him of the father’s stand and the judge would then call the father and discuss with him the grounds of his refusal.
If the judge was not convinced with the reasons of the father, he has the authority to give the brother the guardianship. He also can marry the girl with his authority to that boy.