Police arrested the pair earlier this week, with one of the prosecutors handling the case confirming to regional media on Wednesday that they had been questioned on suspicion of plotting to supply drugs.
Speaking to the Borås Tidning (BT) newspaper, Maria Thorell said that large quantities of illegal substances had been found at the pensioner's home, but declined to give many further details.
“The investigation is at a sensitive stage,” she said.
“We want to try to look for fingerprints or other forensic evidence. There are also phone lists to be reviewed,” she added.
It was not immediately clear if the 20-year-old, who is related to the grandmother on their mother's side, had been living with their elderly relative. However according to BT, police made the arrests after closely monitoring the pair's movements in the area.
Sweden has a zero-tolerance approach to drugs and criminalized the use of illicit substances in 1988.
The move remains popular with more than 90 percent of the population, according to a recent national survey.
Swedish media last year reported on a record number of raids on cannabis farms across the Nordic country.