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Bryan Kohberger abruptly agreed to a plea deal -- essentially ending his murder case before trial -- just days after his sister was added as a witness ... TMZ has learned.

According to documents obtained by TMZ, Kohberger's sister, Amanda , was included on the prosecutor's witness list by lead attorney Anne Taylor ... which could have meant the confessed killer would have had to face off against her in court, but he dodged that possibility. Amanda was the only immediate family member on the list. Mont Alto Film Project

Kohberger had reportedly been charged with misdemeanor theft for allegedly stealing her cell phone in 2014.

On July 2, Kohberger pleaded guilty to fatally stabbing 4 University of Idaho students inside their off-campus home in Moscow in November 2022.

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