At the centre of the case is Takahiro Konishi, 51, son of Tomoko and father of the two girls. In March 1970, as leader of the Red Army Faction, he led one of the most sensational hijacks in history. Nine student revolutionaries, armed with guns and swords, seized a Japan Airlines jet carrying 138 people. After landing in South Korea, they swapped the passengers and crew for a single hostage: the Japanese deputy minister of transport, who had volunteered to take their place. From Seoul they flew to Pyongyang, where they were welcomed by North Korea as heroes and political refugees. The Japanese demanded their repatriation but, lacking diplomatic relations with Pyongyang,they were ignored.
Other Red Army members in Japan were imprisoned for planning the crime but from North Korea almost no news was heard. Then came a remarkable disclosure: three years ago, in an interview with a Japanese newspaper, the late dictator, Kim Il Sung, referred to the hijackers in unflattering terms: "They cannot truly be called revolutionaries, because they live comfortably with their wives and children." The hijackers, it turned out, had Japanese wives, Red Army sympathisers who had smuggled themselves into North Korea via Eastern Europe.
Supporters' groups began visiting Mr Konishi and his comrades and uncovered new surprises: for years, Pyongyang supported them, but recently, as it made twitchy attempts to attract Western aid, its welcome for the terrorists appears to have cooled. In 1990, the government withdrew financial support. The erstwhile student terrorists now run their own travel agency and import- export business, trading with former Communist states.
But North Korea's economyis in crisis: after summer floods and wretched harvests, there are predictions of famine. Since the death of Kim last year, few observers know who commands power. Understandably, all but one of the hijackers now wish to return to Japan.
The wives are wanted for passport violations; the best that the hijackers themselves can look forward to is long prison sentences. But among them they have fathered 18 children, the eldest Mr Konishi's daughter Ritsuko, 18.
Like all parents, they want the best for them. "The children are Japanese," says Yukio Yamanaka, of the Salvation Centre, a left-wing group which supports their repatriation, "but all their classmates are Korean. The education they receive is nationalistic, all about 'our glorious mother- country'. In Pyongyang there is just the Kim Il Sung University; Japan has hundreds of universities to choose from. Their grandparents visit them and tell them about Japan: they just want to visit and see what it is like."
At present, the children are stateless. On her recent visit, Mrs Konishi obtained her granddaughters' birth certificates and this month they were submitted to the authorities with the aim of obtaining passports for the sisters. The case is unprecedented and Japan's Byzantine bureaucracy is sure to take a good deal of time reaching a decision. But Mr Yamanaka is confident that all the hijackers' children will eventually return to Japan.
What awaits them when they do? Quite apart from the inevitable suspicion that they are spies, Japanese society is notoriously intolerant of former exiles. Even children who have grown up in Europe or America often face bullying and alienation when they return to Japan.
There is a tendency, too, to project the sins of the fathers on to succeeding generations. The children of another public enemy - Shoko Asahara, guru of the Aum Shinri Kyo sect, suspected of the sarin-gas attack on the Tokyo subway in March - are facing this problem in their home town, where suspicious parents are resisting attempts to enroll them in the local school.
As a family, the Konishis can live together only in North Korea. "Unless they can all come back together, it cannot be a homecoming in the real sense," says Mrs Konishi. "My granddaughters are innocent, but I expect a lot of difficulties ahead."