Is it permissible for paternal half-sisters to uncover themselves without hijab in front of their maternal uncle and their brothers, due to the existence of rada'a (milk kinship) between some of the siblings, even though it does not exist between all of them?

If you breastfed from your sisters' mother (from your father's side), then their maternal uncle becomes your maternal uncle through breastfeeding, and it is permissible for you to uncover in front of him without hijab. However, if you did not breastfeed from their mother, then it is not permissible for you to uncover in front of him, as he is a non-mahram to you. Similarly, for anyone who breastfed from your sisters (from your mother's side), your maternal uncle has become their maternal uncle through breastfeeding.