South Africa released seven black nationalists from prison today, including the nephew of African National Congress leader Walter Sisulu. The releases raised to 42 the number of political prisoners freed from Robben Island prison since the end of an 11-day hunger strike there in early March. The strike ended after a meeting between lawyers of the 346 hunger strikers and government officials. The details of their agreement have never been made public, but the pace of releases has increased since the meeting. The ANC, the leading black anti-apartheid group, has demanded the release of all prisoners convicted of politically motivated crimes as a condition for its participation in formal negotiations on a new constitution. The prisoners freed today included five members of the ANC and two members of the Black Consciousness Movement. Among the prisoners released was Jomguzi Sisulu, 31, who was sentenced in May 1986 to five years for treason. The father of two small children said he was shocked when authorities told him Thursday he would be freed the next day. ``I couldn't believe it, it was so unexpected,'' he said. Sisulu's uncle, Walter, was freed from the same prison in October after serving 25 years of a life term for plotting sabotage to end white rule. President F.W. de Klerk will hold talks April 11 with top ANC leaders, including Nelson Mandela, to work out a framework for constitutional talks.