I think it's a reference to what Kelmomas calls "the strength"; his Dunyain talents, born out of a thousand years of breeding. If I were to rank them based on that, I think it'd go like:
Inrilatas - Has almost all of his father's strength, but none of his control.
Theliopa - Has a strength close to his father's, while lacking all social faculty. Her intellect distances her from humans.
Kayutas and Serwa - Their closeness suggests to me that they have, by the criteria of Dunyain strength, reach the same level (though Serwa is obviously more powerful due to being a sorceress). Both have inherited a good deal of their father's strength, but it is dilluted by their humaneness.
Kelmomas - Has a good deal of his father's strength, sure; perhaps on the same level as Kayutas and Serwa. Almost as insane as Inrilatas too. I think he is the weakest of Kellhus's children; his invisibility to the Gods is not a consequence of his heritage.