The Cult is really about Isis in my view as she is their fallen Master's actual daughter. As the Master's true heir it is her destiny to assume his holy mission. Granted she again is more of a symbolic figure and Isis herself wants little to do with these devotees and their acts. Sun Tzu is more pragmatic and simply isolates her to the greatest extent possible to their acts on her behalf. Granted their (or rather her) unborn child or children would would have similar power over these zealots. Originally the name comes from the Greek Cult of ancient times and links to real world terrorists are only in a name.
Yeah, this is one of those things where being pedantic isn't going to be very helpful -- the most widely known association with ISIS is the terrorist group. Naming your fictional terrorist group "Isis" is real tacky. Saying "well, that's the Greek name for the goddess!" isn't much of a defense, particularly since the Greek name is a really awkward corruption of Wusa or Aset. Both of those are more 'authentic' to Meroe or Egypt than the Greeks...but they're also completely opaque, which is why I suggest "Cult of Horus" instead. That completely sidelines Isis the character and highlights the astroturf (artificially constructed) nature of this so-called "Cult". Yes, I get that some Exituri and Wobbly zealots believe this nonsense, but it's some brand new nonsense being pushed by the WOB higher-ups and/or Sun-Tzu right after the super-recent assassination of Thomas, not ancient scripture. And Isis herself is all of 18? 19? No way the Wobblies or Sun Tzu are letting her near any kind of power, even if it's supposedly symbolic. You can turn symbolic power into actual power with the right back-up, just ask the Japanese Emperor Meiji.
Who are these powerful League aristocrats?
The dynastic families that control the major multi-world provinces of the Free Worlds League. Like I said, I'm fuzzy on the exact details of the League, but I know the basic idea is that it has a couple of large, multi-world provinces that are effectively run by a major noble house (Halas in Oriente, Humphreys in Andurien, Cameron-Jones in Regulus, Marik in both Marik and Tamarind-Abbey, Graham in Graham-Marik, Stewart in Stewart, etc). And historically, these families have basically controlled the League throughout its history. Most of the independent worlds are one-system provinces, so do not have major dynastic families with the same level of political or economic power as the big provinces.
I think there is widespread general opposition to Isis becoming Captain-General because of many circumstances. The cloud that hung over Thomas before his assassination, her illegitimate birthright, oh and her relationship with Sun Tzu.
I don't think there's a need to be super complicated about why Isis isn't widely popular with the existing power structure. If she was a savvy operator in her 30s, a lot of this opposition wouldn't exist. It's the fact that she's a naive teenager with basically no political experience, and very very very obviously being used as a Trojan Horse by the Capellans, who were the League's biggest nemesis during the only very recently concluded Succession Wars.
Who does this look for a new Province?
Kendall Coalition
Political Leader: Governor-General Helen Montcalm
Capital: Lakeland, Kendall
Number of Systems: 11 (Kendall, Kogl, Stotzing, Tormentine, Lepaterique, Hazeldean, San Nicholas, Atzenbrugg, Huntington, Landfall, Lahti)
Mech Strength: 1 regiment (the Free Lances)
Notes: Officially known as the Coalition of Unaffiliated Worlds this newly created Province centered on Kendall regards itself as the real Free Worlds League. During the opening days of the 3rd Marik Civil War this collection of independent systems formally banded together under the leadership of Jeffrey Marik and Helen Montcalm. According to the natives the Treaty of Kendall resembles the League’s Document of Incorporation as it should be – without any mutli-world members. Each coalition world is represented in the Provincial Parliament with a single member appointed by their own planetary government. While significant legislative powers rest with this interplanetary council strong executive authority rests with the Governor-General in times of crisis. With Kendall being the strongest world in the Coalition it is no surprise that their ruling House Montcalm is the go-to executive office holder here. Militarily the new province is struggling to form an effective defense force as its planetary militias, noble guards, and corporate forces are not that large. As such a new regiment called the Free Lances are being assembled across the Coalition with no single home base. Thankfully its personnel are being augmented by returning FWLM volunteers and trusted mercenaries but it is still a divided fighting force focused on defense spread thinly across the Province. The Lances can rely however on friendly interstellar transport companies for rapid redeployment if necessary, thanks to regularly arranged jump routes set up by the Freebird Enterprises. For now, this arrangement has worked well but if other Parliamentarian forces continue to push onward the Lances will have an increasingly difficult time patrolling their worlds.
I think you're over-complicating this. If Jeffrey leads the "Parliamentary" faction, he doesn't need to have a single large province of physically proximal worlds supporting him. He can have the literally dozens of one-system provinces behind his candidacy, and some of those systems -- like Kendall and Irian -- are monstrously wealthy because of their armaments industries. "Turtling up" in an eleven-world ad-hoc province makes it far too easy to crush his forces with the much larger semi-autonomous forces controlled by the major provinces.
Instead, if you keep his support diffuse, it makes it much harder for the other factions to fight him -- if you hit an isolated independent world too hard, you drive the other indie worlds even closer to Jeffrey. If you leave them alone, he can use them as bases to attack from. This isn't the American Civil War where a portion of the League has gone separatist and there are two clear geographic sides with large set-piece battles -- this is a battle for hearts-and-minds primarily, for every faction trying to get the Captain-Generalcy. The major provinces are just as spread out as the minor ones, and the Concordians don't have a clear base of operations in the League, so you shouldn't think of this as a territorial fight. It's more a bunch of skirmishes, a lot of black ops (small scale Merc insertions are perfect for gaming purposes), with occasional attacks on key targets of opportunity, like Sadurni.
Simpler to explain (and more realistic) for the split to look like:
(a) Big provinces and major noble families: essentially traditionalists that back Paul/Corinne, who have promised them greater autonomy.
(b) WOB, House Centralla, House Liao: all want to subvert the League and take it over from within. Uneasy allies using Isis as a Trojan Horse.
(c) One-system provinces and corporations: want to cripple the Captain-Generalcy and weaken the major provinces.
(d) House Humphreys: traditional role of spoiler, getting dog-piled by everyone else.
That gives you a very easy fit to the English Civil War (faction-for-faction) and you can easily crib stories from real history -- even battles from real history -- to create detail for the Civil War.
If you really want a "Kendall Province", my suggestion is to make it diffuse and hard to pin down, centered around all the major corporate worlds, and a random selection of others, but very few of them "officially" supporting Jeffrey, which makes it much harder for the other factions to move against any of them decisively, for fear of losing support in the Civil War.