The SEx came out a four months after the IIx and two months prior to the IIcx. A check of the ASICs on the SE/30's MoBo against the IIx and IIcx Block Diagrams in the DevNotes would be the definitive DNA test for the heritage of the SE/30. My money would be on the IIcx if there was any ASIC evolution during the downsizing of the IIx to the IIcx and SE/30 form factors.

The SE/30 appears to be of the same development cycle as the IIcx, so I guess the SEx and IIcx might also be viewed as fraternal twins.