In the first case of its kind, the widow of a former Rwandan supreme court judge and the sister of a Tutsi former cabinet minister say the UN failed to stop the murders during the 1994 Rwandan genocide in which 800,000 people were massacred.
They accuse UN soldiers, who were supposed to defend the Tutsi people, of either handing them over to the Hutu killers, or of running away.
Anonciata Kavaruganda and Louise Mushikiwabo are suing for damages, although the amount sought has not been revealed.
Robertson, of Doughty Street Chambers is running the case with former Australian crown prosecutor, Michael Hourigan, who left his job as an investigator with the UN's International Criminal Tribunal for Rwanda in protest against the UN's inaction and refusal to cooperate with his investigation.
Hourigan says: "We're really in a new field, this has never been done before. At the moment, I guess it's just going to be done by way of first speaking to the UN and getting a sense of how they would want to negotiate nor arbitrate with the families.