She had planned to stab people around the Palace of Westminster in central London last year, while her 44-year-old mother Mina Dich admitted aiding her plot.
The scheme had been hatched by younger sister Safaa Boular, 18, who wanted to target crowds at the British Museum but had been arrested and remanded in jail over her attempt to become a jihadi bride in Syria.
Following her detention the siblings shared phone calls in which they discussed holding an "Alice In Wonderland" themed tea party that prosecutors said was code for an attack.
The security services launched a proactive surveillance operation, and in April 2017 tracked Rizlaine Boular and her mother as they conducted reconnaissance of London landmarks earmarked for attack.
Days later, officers swooped to arrest the mother and daughter in an operation that led police to shoot Rizlaine Boular, who made a full recovery.
The family members, including Safaa Boular, were charged with preparing terrorist acts.
She denied the charges but was found guilty by a jury last week and will be sentenced at a later date.
Rizlaine Boular pleaded guilty earlier this year and now faces life in jail, with a minimum term of 16 years.
Their Morocco-born mother Dich was handed a minimum of six years and nine months plus five years on extended licence. Judge Mark Dennis said she had "failed in her parental role" and played a "significant role" in radicalising her daughters, bearing a "heavy responsibility".
Dennis told Rizlaine Boular she had planned a "wicked act" born from distorted views.
"There are individuals walking the streets today whose lives could have been irreparably damaged or lost had it not been for the intervention of the police and security services," he said.
The defendants, who wore Islamic dress in the dock, made no reaction. Boular's lawyer Imran Khan said his client's plot was a suicide mission, because at that time she had wanted to die, but had now put her past views behind her. Kieran Vaughan, a lawyer for Dich, argued she played a "lesser role" in the plot.