According to Wikipedia, and Geni.com, the British usurper Carausius c. 286 is the grandfather of Magnus Maximus c. 383. This would chronologically make sense, but is a little bizarre. From what I understand, Carausius was a Menapiian (Belgian). I would like to find another source. Maximus' father is listed as Flavius Iulius Eucherius. His uncle was Theodosius the elder. https://en.wikipedia.org/wiki/Magnus_Maximus
Well, I found the tentative link. According to some online genealogies, and repeated in the book The British Chronicles, Volume 1, by David Hughes, pg 128 states (emphasis mine) that:
It is generally accepted that Maximus (Macsen "Wledic") was the eldest of the three sons of the Roman Governor Eucharius(355-8 & 371) and his wife Flavia, daughter of the earlier British usurper Carausius II (353-356).
The book also discusses two other lineages, but this seems to be the one relevant to the question.
It appears little is known about this Carausius II :
Carausius II is the name given by historians to a possible imperial usurper in Roman Britain between the years 354 and 358. Coins appear during this period bearing the name which is the same as an earlier British usurper emperor, Carausius.
So it appears Maximus was descended from a Carausius, just probably not the Carausius (Marcus Aurelius Mausaeus Valerius Carausius) indicated by wikipedia.
This Wikipedia article lists the relatives of Count Theodosius:
Magnus Maximus is not among them.
According to Wikipedia article on Magnus Maximus his full name was Flavius Magnus Maximus Augustus. That seems a rather odd name for someone whose father was named Flavius Iulius Eucherius. Byt in late Roman times it was common for two brothers to have completely different names, each selecting a few names from the many their ancestors had. It does make him son of Flavius Eucherius and the daughter of a Carausius.
I have the suspicion that some of the relatives attributed to Magnus Maximus were not his real relatives.
the family tree of the dynasties of Valentinian and Theodosius does not show Magnus Maximus.
So I am somewhat doubtful about those alleged relationships of Magnus Maximus.
This thread discusses the genealogy of Magnus Maximus.http://historum.com/ancient-history/114645-correct-dates-constantius-chlorus-magnus-maximus.html3
Here is a link to a family tree with a conjectural relationship with the family of Theodosius.enter link description here
Carausius (Caron,River is named after him, it was a Roman Terminus he used) He was the son of Athirco, king of the Scots, and was the brother of Donald II, and Findochus, He was expelled by his family, he joined the Roman Army and Navy and returned to Falkirk with a Roman Fleet, he created a peace between his Scots Family and the Picts, he was forgiven for his betrayal by his brother Donald and Findochus. Falkirk and Central Scotland in the East is called Pictland and where ruled by the Gothic Hun (Picts) from Camelon (a bend) their City on the Carron, Pictland or God[th]oddin. [Vot/Goth/Got/God=The] "The Oddini" The Tribe of the Gothic Hun, (Cruat-Hun or Kraut-Hun) Migrant agricultural seasonal Workforce for the Roman Empire and Others national Groups including their own.
Also Carausius is from the Irish-Scot (Gwiddel-Scots) or Gales who ruled the Isle of Man (called Manapii in the beginning) as with Manaw (Manapii) Gododdin, which, to is, because of the tribe of Marini, who used Alatum or Edinburgh Castle at one time because they where Gaelic seamen who came from Manapii in Belgium and they to where an Oddini tribe even Brutus and his people came from Tarhuntaussa (Taur-us [Mt’s] -Hun-taussa) of the Wilusia Troads, the tribe of King Walum who ruled, over the Trojans, and were an anti Hitti federation tribe, Trojans from Walum down through to Aeneas and Lavinia, daughter of King Latinus I, in whose time Rome was built, then down the line of Silvius of Alba Longa and then Brut-us (B/Crut-us -Hun).