Who is the guardian for a new Muslim woman who has no male guardian other than her Christian grandfather and her imprisoned Muslim brother, given the presence of Muslim uncles in other countries?

A woman's guardian (wali) is her father, then his father (paternal grandfather), then her son, then his son (paternal grandson), then her full brother, then her half-brother (from the father's side only), then the sons of her full brother, then the sons of her half-brother (from the father's side), then the paternal uncles, then their sons, then the paternal uncles of the father, and then the ruler. Your guardian is your Muslim brother. The fact that he is imprisoned does not prevent him from being a guardian; he can appoint someone to conduct your marriage contract. If that is not possible, then your guardian is your paternal uncle. His being in another country does not matter, as he can appoint someone to conduct your marriage contract or the contract can be completed through modern means.