16-year-old Becky Watts disappeared from her family home in Crown Hill, Bristol in February. Her body parts were found in a garden shed several days later.
Becky's stepbrother, Nathan Matthews, 28, and Shauna Hoare, 21, are accused of conspiring to kidnap, and murdering, the her. They deny the charges.
The pair shared an "unnatural interest in attractive teenage females", a jury at Bristol Crown Court has heard.
Opening the case, prosecutor William Mousley told the jury that Becky was killed in her bedroom.
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"She was suffocated, despite her fighting for her life. There followed a deliberate, carefully planned and grotesquely executed plan to cover up her killing," he said.
Becky vanished on February 19 and was reported missing the following day. After a huge police search, her body parts were discovered in a garden shed at a house in Barton Hill, Bristol, on March 2.
Another couple, who stored the dismembered body in their garden shed, have pleaded guilty to assisting an offender, the BBC reported.
Karl Demetrius, 29, and his girlfriend, Jaydene Parsons, 23, admitted the charge at an earlier hearing at Bristol Crown Court.
However, they told the court they "did not know or believe" the bags contained Becky's body.
Donovan Demetrius, 29 - Karl's twin brother - and James Ireland, 23, are also charged with assisting an offender.