Complex sentence: One of the above contestants, Şebnem Paker and her group, had performed in Eurovision Song Contest 1997 and had received the highest points for Turkey up to 1997, but in 1998 they couldn't repeat their previous success.
Split into simpler sentences: One of the above contestants, Şebnem Paker and her group, had performed in Eurovision Song Contest 1997 and had got the highest points for Turkey up to 1997. But in 1998 they couldn't repeat their previous success.