Two Sri Lanka Freedom Party (SLFP) activists loyal to the former president Rajapaksa had filed a case in 2016 saying that Sirisena taking over from him was against the party constitution.
They said proper procedure had not been followed in Sirisena's succession as SLFP leader.
Rajapaksa handed over the party reins to Sirisena immediately after he lost the January, 2015 presidential election to Sirisena.
Sirisena, who was Rajapaksa's health minister, had broken away from him to become the common opposition challenger to Rajapaksa.
Rajapaksa earlier this month publicly claimed that he was still the leader of the SLFP.