Terror plotter's family in Sydney court

The sister of an Islamic extremist who pleaded guilty to planning a terrorist attack has told a Sydney court her family is still trying to comprehend his crime.

Family and friends of an Islamic extremist who plotted a terrorist attack have expressed their shock and disbelief in a Sydney court as to how their "naive" boy ended up behind bars.

Sulayman Khalid, 22, Jibryl Almaouie, 23, and a teenager who can't be named for legal reasons, have each pleaded guilty to conspiring to do acts in preparation for a terrorist act to advance "violent jihad" in late 2014.

"It's just too much to even comprehend - the fact that he's even charged with anything violent let alone violent extremism," Khalid's older sister Maryam Khalid told the NSW Supreme Court in Parramatta on Wednesday, the second day of his sentencing hearing.