win

thanks

I haven't tried yet, but if I did try, I would probably pull back a few men to be ready for the sisterhood. I would also focus most of my men on a single one of the two enemy groups and try to destroy it so I could target the other.
During one of my playthroughs I saw the villager confederation blitz the sisterhood's capital before the Ominous Legion could even integrate/puppet them.

Focus on defence in the south and capture the northern port to cut the northern groups supply line

commit terrorism against the enemy

Personally, I won on my first try; Here is how I did it, but it might not help you a lot since I'm not very good at explaining, but it's worth reading:

Split your starting army, 7 in the south, 7 in the north. You'll want to focus 90% of your attacks on encircling, I managed to encircle and kill 20 divisions of their army in 2 years which gave me a massive advantage.

You want to train more units, wouldn't train less than 5 or more than 8 (i did 7). With the extra units, use them to hold the line at the Sisterhood when they become independence.

Once the south is gone, You'll have an insanely good advantage in the north.

Combine your armies so that you have an even number of divisions in 2 armies fighting on two different sides; chances are the Ominous Legion and the Sisterhood will both be in the north, so this is why.

Once you have gotten them out of your mainland:

Research a Battleship hull, and if you have Man the Guns, research a Battery and Transport (I think the first Transport tech was called Convoy Escort, correct me if I'm wrong)

Also hire a naval advisor, you'll need 10 naval xp.

Once the navel preparation is done, make a ship design with just a Battery and Fire Control, then produce 1 ship.

Once that ship design is done, produce the ship. In the meantime, prepare a naval invasion with an army. Once the ship is deployed, order it to go to the Yellow Sea (assuming you are attacking their capital island). Once you have naval supremacy and the naval invasion ready, launch it.

Then, just take their capital and they should capitulate.

I'd puppet whatever members joined their faction (besides the Sisterhood) since you can get some manpower and territory.

Hope this helped!

Put divisions on all the sisterhood victory points before it breaks out, you can then instantly capitulate them, and you'll only have to deal with the ominious legion. Update your divisions to make them less terrible, and get enough divisions to push them off the home islands. Then just prepare a naval invasion to finish them off.