The couple had hoped that killing the woman's parents would allow them to cash in on a life insurance policy for which the 26-year-old woman was the sole beneficiary.
The Swedish pair had already bought one-way tickets to Buenos Aires and planned to flee Sweden after carrying out the plot, the Metro newspaper reported.
The young couple drugged the elderly pair with Sobril in their apartment in Solna near Stockhlm in October, ransacked the flat for valuables, and then set fire to the building in seven separate places.
The hope had been to inherit a summer cottage, life insurance and the apartment.
However, an attentive neighbour noticed the smoke and sounded the alarm, allowing rescue services to save the elderly couple before they were harmed.
The young couple was also charged with giving false testimony after giving police faked witness statements, in which they stated that a relative of the girlfriend had previously threatened to kill the parents.
The 28-year-old man stood as a candidate for the Green Party at the 2010 election although the party has since stated that once the man's views became known steps were taken to expel him.
Last week, the man and woman were found guilty by Solna District Court.
The couple continues to deny the charges and is set to undergo psychiatric evaluations ahead of sentencing, which is scheduled to take place next month, according to Metro.