Do the children of a son who died during his mother’s lifetime inherit, and does the full sister inherit one-third of the estate, and the daughters two-thirds, and is the mother considered a kalalah due to having children of her son?

What was mentioned is incorrect. The four daughters receive two-thirds, and the remainder goes to the three grandsons (sons of the son) by ta'sib (residuary inheritance). The full sister receives nothing because she is completely excluded by the grandson. The term kalalah refers to the absence of ascendants or descendants. It is important to note that the inheritance should not be distributed hastily until the heirs are correctly identified.