It is possible that we have the names somewhere of Sigibert's real father and grandfather scattered in the records of the fall of the empire. One has to be caeful with Gregory's time scheme (often he was not sure) but it appears that the story is probably set around 500 before Alaric fell (The Ripuari and Alemanii were two of the "peoples" that Clovis overcame to cause Alaric to want to make peace with him before the final battle. Sigibert will have been born ca 440 (a few years either way) as he was "old" when Chloderic killed him. Clholderic was probably in his thirties before he felt he had a sufficient following to try such a stunt and if his father were about thirty when he was born (statistical guess) then that would be a birth date of Sigibert around (or in the decade beffore) 440. That would suggest that Sigibert's father's floruit was about 450 (when the great barbarian coalition was assembled by the Romans to fight the Huns). His grandfather would have been about 420 (born 390 or so) and would have been involved in the matters surrounding the rise and fall of Constantine and Iovinus, the invasions of the Visigoths and the rise of Aetius. I admit it is not certain the names are recorded or that they were yet in any position of authority in Cologne. We know from Sidonius that at Augusta Trevorum (Trier), the family of Arbogastes the Frank (involved in the troubles of the late pagan usurper Eugenius) was installed as count (470's). That suggests at least three powerful Rhenish Frankish families in the late 400's - the "Ripuari", the "Arbogastes", and the "Chlodomeri". Anything further up the River belonged to the Alemanii or whomever Clovis placed over them. It is also probable that the Frisii remained independent down the River.
The Franks and other Germans -probably families of men who had been prominent in Roman service- began to organize into Roman style familia in the late 4th century. It was during this era that two great Gothic clans appear and additionally, the success, if temporary, of the Vandal Stilicho, the Franks Marcomir and Theodemir and the first Arbogast all encouraged this (and the Burgundians being progressive may have already been doing this as well). By the dawn of the fifth century, the families of these military leader clans among the Vandals and Burgundians (and probably the Rhenish Franks) had begun to intermarry extensively so that by the fall of the empire in the West there was a tightly bound barbarian network of kinship among German Generals and later kings. Theodoric the Ostrogoth used and even expanded on this. Cologne (Colonia CAludia Ara Agrippinensium) was not an unimportant town in elaher the Roman or German organizations. One day someone may unearth a tomb and an inscription may tell us more.
It is interesting that there was a habit among the Germans of this time that when a certain "Germanic" people beccame used to the rule of one of these families, usurpers and conquerors tried to identiify with it. There were several ways in which this was done (and it is well attested from the Merovingian era) but one of the more interesting effects that may be of use is the alliteration of names. Among the Saxons in Britain (who for most of the "pagan" period were vassals of either the Nuestrian or Austrasian Merovingian king) The earliest rulers in "Wessex" all began their name with the letter "C". The Nortumbrians for a time liked "O". The kings of Wessex, when they were not affecting the names of Merovingian nobility used "A" (Aethelbert, etc) and the Wessex clan (when it has been exiled to Kent and married into some Kentish royal blood) dropped the their old preference for "C" and adopted the "A" of the Kentish royal house.
The most interesting use is the letter "S" only ever used by the Royal house of Essex in Britain and only with the element "Sige" (Sigeric, Sigebert etc). This name had been adopted by Lothar for his son "Sigebert" who became ruler of Austrasia (which led to some speculation in the peanut gallery that Ingund, Sigebert's mother may have been neice of King Sigibert of Cologne- they were doubtless related but perhaps not in that way) The Royal clan of Essex in Britain first appears in the 570's. They were pagan and very unpleasant toward Christians in Britain so Pope Gregory sent St Augustine to covert them.
The Saxons who settled in Essex probably did not settle there before the 570's and were almost certainly part of the overflow of a band of Saxons who had gone into Italy (with Austrasian connivance) with Alboin king of the Lombards. When Alboin died and the Lombards experiencced setbacks they sent many of these warlike Saxons home. My guess is that Thudebert or Theudebald had put one of his relatives (Sige something - see the parallel example of Theuderic's cousin Sigivald who irritated the people of Clermont so mightily). When Theudebald died, the Saxons at first refused to pay homage to Lothar and there was a war. It may be that in typical Merovingian fashion Lothar gave Sigebert his name at this time and made him a "king" over Austrasia to placate the restive Ripuarians and Saxons.
Anyway, these Saxons returning after many years and finding nothing to do in Saxony (and not much inclined to farming) went to Britain where the only place Aetehlburt's father, Eormenric (Saxon version of Gothic heroic name Hermanneric) who was then King in Kent could send them was against the British in Verulamnium and possibly (still) in London. It is possible that the British polity (called by scholars now "Calchvynned" or chalk mountain though I prefer "Verulamium")was already to some extent partially "Saxonized" in culture if not in blood - they appear to have incorporated communities of Saxons to defend themselves - and was and had been for many years at peace with Kent. It is not clear why Eormenric would war with them except that this is the period when Kent became briefly the dominant kingdom in Britain, changes in the situation on the continent with Chilperic and his wife Fredegunda meddling with the Saxon/Breton colony at Bayeaux (it may be from this time and not 495 that the "Credicingas" invaded Hampshire. In my other post on this site I mention Wessex dating and recent work on that) and Eormenric may just have had a lot of warlike Saxons dumped on him that he had to find homes for. Or there could have been a crisis or breakdown in whatever passed for leadership at Verulmium (We are completely in the dark about all this though better off than a century ago).
Though there would be five or six generations unaccounted for, the kings and later "earls" of Essex (pre conquest) may have been descendants of the Kings of Cologne, just as the kings of Kent were [verifiably in this case] descendants of the Merovoingians (cf Gregory of Tours Book IV 26)Cologne. I emphasize that though I think the possible desccent of Essex is intriguing, there is no very solid basis now for saying that the Earls of Essex were descended from the Kings of Cologne - and one can and should I think be able to speculate all kinds of things as long as he makes it clear it is speculation and does not thereby trouble someone else unduly.
Anyway, what is the old saying "From they to whom much is given, much is expected" We have a lot of blank areas. We have been left a lot of information that, interpreted rightly may fill some of these blank areas. We have had good people going at it for a long time (plus a few wrong turns but it would scarcely be any fun otherwise). The sad thing here is that the main thing we probably remember poor Depoin for (and he gave much that was good) is an error he would probably now be the first to admit and correct. It is just one of those things.
Anyway, I will let it go with that. Sorry to run on so long again. I am glad people raise such interesting questions. I particularly profited from the opinion of the poster whose answer to you I first responded directly to and from the chance to read again Mr. Olar's post at the newsgroup.