MIAMI, Jan. 10 (AP)--Juanita Castro, 24-year-old sister of Fidel Castro, Cuban rebel chieftain, smuggled herself into the United States from Mexico a week ago and has asked for political asylum, her attorney said today.
    Ralph H. Aquilera, the attorney, said the State Department had canceled her United States visa while she was in Mexico and "because of threats to her life" she had returned, though illegally.
    "She secreted herself in the trunk of a taxicab for the crossing," Mr. Aquilera said, and "went to San Antonio where she took a plane to return to Miami.  She surrendered to the Immigration Service today."
    Immigration officials said a deportation hearing would be held Jan. 22 but otherwise declined comment "until an investigation is completed.