There have been at least two groups named Supersister, the most famous one being a Dutch progressive rock band from the 1970s. The other one is a British girl group.
1. Supersister was an experimental rock band from The Hague, The Netherlands, that existed from 1968 to 1974. They played a jazz-tinged brand of progressive rock. Classic line-up: Robert Jan Stips (keyboards, vocals), Sacha van Geest (flute), Marco Vrolijk (drums) and Ron van Eck (bass).
The band started as a school band (Sweet OK Sister) in 1968, featuring singer and songwriter Rob Douw, who soon left the group. The remaining members continued as a more serious musical quartet under the name Supersister. Their style was Canterbury Scene-style prog rock, with a dominant Stips as the group's keyboard virtuoso.
Their début album, Present from Nancy (1970), spawned their first (and biggest) Dutch hit single, She Was Naked. After two more studio albums (1971's To The Highest Bidder and 1972's Pudding And Gisteren) both Van Geest and Vrolijk decided to quit.
The remaining members, together with newcomers Charly Mariano (wind instruments) and Herman van Boeyen (drums), released the album Iskander (1973), a jazz-rock concept album based on the life of Alexander the Great. In 1974 Stips and Van Geest re-united for a final album (Spiral Staircase), released under the original band name, Sweet Okay Supersister. It turned out to be the group's swan song. Robert Jan Stips moved on to play keyboards in Golden Earring for nearly three years (1974-1976).
The classic 1970-1973 line-up reunited in 2000 for a performance at the Progfest in Los Angeles and a short tour of The Netherlands. To mark the occasion a rarities album was released: M.A.N. featured live and studio recordings from 1969-1973. The reunion abruptly came to an end when Sacha van Geest unexpectedly died in the summer of 2001. The reunion concert at the Paradiso in Amsterdam was recorded and later released on CD (Supersisterious, 2001) and DVD ('Sweet OK Supersister', 2006), which also features several old and new documentaries, photographs and unreleased audio tracks.
2. Supersister was a three-piece girl group from the United Kingdom. They had hits in 1999 and 2000 such as 'Coffee' and 'Summer Gonna Come Again' (since covered by the band Nylon).