The judge, Mr Justice Garnham, described Lavrynovych as a “useful idiot” who could be easily manipulated.
He said he had been “easily bought” and “accepted the job as you had accepted other grubby little tasks”.
Carpiuc played a “supporting role” in the attacks, which the judge branded “utterly reckless”.
The court heard a Toyota once owned by Sir Keir was set on fire in a street in north London in May last year.
Days later two homes were set alight, including one rented out to the prime minister’s sister-in-law Judith Alexander, which he still owned.
She told the trial of the billowing black smoke going up the stairs while she, her daughter and partner were in the house.
Lavrynovych and Carpiuc were found guilty at the court on Monday.
Lavrynovych was also convicted of alternate counts of damaging property by fire being reckless as to whether life was endangered. He was acquitted of damaging property by fire with intent to endanger life.
A third man, Petro Pochynok, 35, was found not guilty of the conspiracy charge.