Aha!
So if Karla's maternal grandparent's were not Jewish, then it seems that there are two possibilities: (1) Her mother Julia become Jewish by conversion (conversion to Judaeism is recognized as effective but conversion from Judaeism is not, in the matrilineal tradition, IIRC.). Does anyone know if Karla's father is Jewish, which (amongst possible reasons) would be a possible reason for Julia to have converted to Judaeism? (2) The only other possibility would seem to be that a female forebear ("X") of Karla farther back than Eleanor Renwick, nee Marco was Jewish, such that Elanor Renwick, nee Marco and any other forbears in the female line descended from "X" (including Karla) would be regarded as Jewish too, in the matrilineal tradition.