Many here may know that Classical Latin had no word for “Yes.” The most common used was vero, truly. Latin teachers have often resorted to teaching schoolchildren, ita vero, (thus truly) when they were asked how to say yes. Some claim est - it is, but there is no evidence of Latin speakers using this in the pure sense of yes.
In vulgar Latin (my 9th Grade teacher had a marvelous term to explain vulgar Latin, “Not what Cicero would write in a carefully crafted bit of oration, but an exclamation to be uttered by a guy sweeping the horseshit off the Appian Way”), there were several work arounds: sic – thus or so – gave rise to si in Italian and Spanish; hoc ille- "this is that” – was popular in Gallic regions. Through a series of sound changes it became French oui. In southern France, it retained the hard “c” to become oc. That in turn supplied the name for the dialect, Languedoc (lenga d'òc ("language of òc") or Occitan.
The first literary reference I have found to this set of terms is from the Italian poet, Dante in his De vulgari eloquentia (On eloquence in the vulgar (vernacular) c. 1302), he wrote in Latin, "nam alii oc, alii si, alii vero dicunt oil" ("for some say òc, others sì, yet others say oïl").
The English word “yes” comes from Old English, gise or gese – so be it, from proto-Germanic, sijai, from proto-indo-european, si, a form of the ultimate root, es, to be. This in turn evolved into the Latin verb “to be” sum – I am, es -you are, est – he she or it is.
So, by a convoluted path, we see that es in Latin most closely resembled in its origin yes in English. They sound the same, but are only distant cousins, not parent child, nor used the same way.