Is it true that the younger brother, whose father has passed away, claims to be the guardian of his elder educated sister, and that obedience to him supersedes a wife's obedience to her husband? If so, how valid is this claim?

For a brother, guardianship over his sister after the father's passing is for her maintenance and care, not for domination. A wife's obedience to her husband takes precedence over her obedience to her father, so how about anyone else? Mutual respect should prevail between siblings. The brother should respect his sister due to her right in Islam, their kinship, and her elderliness (if applicable). It is advised to be patient with the brother, supplicate for him, advise him gently, or let someone influential advise him.