How should I act regarding the problem of my father's desire to marry me to one of my female relatives whom I consider like a sister, knowing that he previously disassociated himself from my siblings for the same reason?

A state of permanence is impossible. To solve this problem, first consider marrying your relative. If she possesses the qualities of a righteous wife and has no bad characteristic that you cannot endure, then marry her. Perhaps Allah will decree good for you, and your dutifulness to your father will be a cause for your marital happiness. However, if there is a defect that prevents marrying her, explain that to your father. Try to convince him that forcing a man to choose a wife he is not inclined towards may lead to the failure of his marriage and harm the girl. Advise your brothers to be dutiful to your father to strengthen relationships and mend what has changed.