When the all-Ireland CPI dissolved in 1941, Sinclair remained an active member of the Communist Party of Northern Ireland (CPNI) and served as its Secretary from 1942 to 1945.

Republicans sought a chance to respond to this, and the Red Hand allowed Jack Brady to write an article voicing their views.

However, the IRA was a proscribed (banned) organisation at the time in the United Kingdom and the Republic of Ireland, and publishing their material was illegal.

As part editor of the party, Sinclair was held responsible for the article and later interned, originally for a two-year prison sentence, but this was reduced to two months on appeal.

However, he was denied a passport by the US State Department due to a long history of supporting left-wing politics and a pro-Soviet Union stance, as well as an anti-colonial attitude.