if you are going to play Sturgia - you need to lvl up as fast as you can to prevent this.
That being said typically after some time Khuzait will be back to fighthing vs Norhtern/Southern or Asarai, and thats when you pounce.
Imo overall the siegepreperation take way to short time in this game. 24hrs or less lets go..
Vlandia will usually stop when thye conquer Mazhdan + Kraniog + Varcheg or before even.
Specially if they get involved with Battania or Western Empire or both.
Ideally if you want to lvl up your clan to be eligible to be a merc to prevent the "instant loss of Tyal + Uriksala castle and maybe even Vladiv"(and if its really horrible even Sibir + Varnapol) I suggest just running around the map to take out bandit parties, or if you are skilled enough do Arean tournaments in towns, nice loot + denars + atleast 3 Renown pr win, which isnt all that many wins before you hit clan tier to be merc.
Also focus on Steward to bolster how big and army bonus you can get from that(cause defending Tyal with 50 tropps + garrison isnt exactly easy when the enemy is 1400+ strong).
Khuzait just moves to fast on the map imo, even with the new nerf to cav on snow.
Sturgia is poor to start out, so their defenses is also crap.
Usually if they are in a 2front war vs Vlandia + Khuzait - I focus on twarting Khuzait myself, so typically the rest ends up stopping Vlandia only looseing the castle which is "literally" in Battanias backyard.
The game goes abit back and forth typically anyways.
But some factions just have the deck stacked against them tbh imo - Battania is typically gone in my plays by year 5 or 15 at most or may be left with 1 holding(Pen Cannoc most of the time).
The war/peace mechanics in the game is just garbage in vanilla imo, the ai is just predatory and cowardly(ganking all the time).
That being said I'd suggest if you play on pc to use mods ->
Diplomacy will let you put in WarExhaustion, so that snowball cant go on forever.
Vanillaplus adds in some other nice features.
There is many others, but for me haveing those in the game and tweaked right just makes the game that much more fun.
Slows down the the game, so there is a purpose to haveing "dynasty aka kids".
Haveing less wars etc also means that factions can make a huge comeback.
Vanillaplus increase that alot, since rebelling cities typically end up back in the hands of the original faction, and the rebels end up being a normal merc clan for hire.
There is much more rebellions overall in the game(so be smart about suggesting policies that counter that)
I just wish they had stuck with the old causis belli so we wouldnt have the issue of ganking and warmongering that plagues the game, it just tanks the fun out of the game when its just endless/constant wars going on.(again the mods you can tweak settings so peace last much longer so you can actually have periods of no wars even).