Is it permissible to marry the children of a maternal uncle (khal) or vice versa, given that the mother's mother breastfed the sister on a maternal uncle, and the maternal uncle's son was breastfed by the younger maternal aunt, and is it permissible to marry the children of the maternal aunt who did not breastfeed anyone from the grandmother?

It is not permissible for your sister, who was nursed by her grandmother, to marry any of her maternal uncles' sons (cousins) or maternal aunts' daughters (cousins), because they are her siblings through suckling. This is based on the Prophet's (peace be upon him) saying: "What is forbidden by lineage is also forbidden by suckling."

Similarly, your maternal uncle's son, who was nursed by his grandmother, is not permitted to marry any of his maternal uncle's daughters or his maternal aunt's daughters, because they are his sisters through suckling.

As for you and the rest of your siblings who were not nursed by your grandmother, and likewise your maternal uncles' sons and maternal aunts' daughters who were not nursed by their grandmother, there is no objection to marriage between you.