His sister, a mother of four, had burned herself to death to protest China's 'violent rule' in Tibetan areas.
Taken into custody by Chinese police in June, he has not been heard from since.
He follows writer Gangkye Drubpa Kyab into detention after a public display of respect to the Dalai Lama.
Gangkye Drubpa Kyab is taken into custody after attending a reception welcoming his early release from prison.
Tibetans attending the Northwest University of Nationalities in China's Gansu see themselves marginalized by new policies.
They were convicted of sending information and images of a Tibetan mother's protest to outside contacts.
Monks and nuns from Qinghai, Gansu, and Yunnan provinces are told to leave the huge religious study complex.
He is freed without explanation after serving more than four years for criticizing Beijing's rule in Tibetan areas.
Usually held only by monks, the event draws hundreds of nuns from seven convents in Sichuan and Qinghai.
A resolution of the dispute over Tibet's status has been hindered by separate security concerns, a Tibetan scholar says.
Both monks and laypeople take part in religious instruction and promotion of Tibetan language and culture.
He had watched a program in the prison's TV room outside permitted hours, a local source says.
He joins another Socktsang monk in custody after being seized at gunpoint in his quarters.
Sentenced for having supported a self-immolation protest, he is freed for 'good conduct' while in custody.
Described as involved only in the monastery's work, he may have been targeted for viewing news broadcast from outside areas.
He is warmly greeted at home by relatives and friends despite police warnings that no welcome should be given.
Lobsang Kelsang's family still lack information on his condition, though, and are not allowed to visit him.