This term conveys neither positive nor negative connotations and is often used in objective narrations. However, using it to describe someone the speaker knows personally is often considered lacking respect, where more polite forms like お姉さん(onēsan) are preferred.
Matsuoka Shizuo (1929) 日本古語大辞典[Comprehensive Dictionary of Archaic Japanese] (in Japanese), アネ(姉), page 56: “之からアニといふ語が分派した ― The word ani developed from this”