Is it permissible for me to sit in front of my female cousin without a head covering, knowing that my female cousin and my sister are milk-sisters?

If the sister has been breastfed by the maternal aunt, then the maternal aunt's son becomes a foster brother to the sister, and he is permitted to see what a man sees of his maharim. As for the questioner, she is a stranger to her maternal aunt's son unless she was breastfed by his mother, because the ruling of breastfeeding is specific to the nursed individual alone, without his or her siblings.