Is my marriage valid in Islamic law, given that my father initially opposed my marriage but later accepted it?

Fathers must take the initiative to marry off the women under their guardianship if a suitable suitor proposes and the woman consents to him. If a guardian prevents a woman from marrying a suitable suitor when both the woman and the suitor desire it, this is considered forbidden 'adhl (unjust prevention).

If the closest guardian is unjustly preventing the woman, the guardianship transfers to the more distant guardian, and it is permissible for him to marry her off. However, if the more distant guardian marries her off while the closest guardian exists and is not unjustly preventing her, then the marriage is invalid.

Scholars have differed on the ruling regarding the marriage performed by an unauthorized agent (one who acts on behalf of another without legal permission): some consider such a marriage void and unaffected by the guardian's approval, while others believe it is valid but contingent upon the guardian's approval.

Therefore, if the husband is suitable and the father rejects him, the father is considered to be unjustly preventing the marriage, and it is permissible for your brother to marry you off. However, if the husband is unsuitable (e.g., due to his فسق - open disobedience to Allah) and your brother marries you off, then the father approves the marriage, the contract is valid according to some scholars. To avoid scholarly disagreement, the marriage contract can be re-established in the presence of your guardian (the father), the husband, and two just witnesses.