It is not permissible to marry one’s foster cousin, if they were breastfed by the same women in infancy, i.e. till the age of two. However, if the foster brother and sister got married unknowingly, the marriage is null and void. The husband should verbally terminate the union. (Fataawa Mahmodiyah vol.10 pg.344)
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai