andrew steele , 40 , claims that the terminal disease affected his brain .
steele is charged with first-degree intentional homicide for the murders of his wife , ashlee steele , 39 , and her sister , kacee tollefsbol , 38 , in 2014 .
he has plead not guilty by reason of mental disease or defect .
jury was presented with steele 's letter written before slayings claiming he had had threesomes with ashlee and kacee .
his attorney said steele 's last memory from day of killings was looking for scissors to cut zip tie from around his wife 's neck during bondage sex .