I looked up the word in Ásgeir Magnússon's etymological dictionary.
It seems my guess on the grades may have been right on. The Vedic
relative shows zero-grade as well in /satyá-/. (In Vedic the syllabic
/n/ developed into /a/.) On the other hand the Latin word (sons, sontis)
seems to show the o-grade - just like Old Norse and West Germanic!