The pre-dawn bombings in the northern city of Thessaloniki on July 1 targeted the homes of members of Greece's governing conservative New Democracy party.
The 72-year-old mother of parliamentary candidate Afroditi Nestora was killed after a crude bomb made with camping gas canisters exploded under Nestora's car, which was parked at the entrance of her apartment building.
Nestora herself suffered burns and remains hospitalised, but briefly left hospital Thursday to attend her mother's funeral.
Her father and two residents of the building were also injured.
Two attacks on the same night targeting other New Democracy party members caused damage but no injuries.
Greece has a long history of politically motivated violence dating to the 1970s, with domestic extremist groups carrying out small-scale bombings, usually targeting symbols of power or the property of politicians, police or other authority figures.
While the groups most active in the 1980s and 1990s have been dismantled, new groups have emerged.
Police said on Friday they arrested a 29-year-old man in the northern city of Thessaloniki and a 26-year-old woman on the southern island of Crete on suspicion of involvement in the bombing at Nestora's home, as well as one other man suspected of hiding the two in his apartment before and after the attack.
Police said the search for others potentially involved in the series of attacks continued.
