- Althea xor Thais must be the mole.
- You must choose Vonne as your queen and make her crazy jealous (see the Yandere Principle).
- Because you will be relying so much on Vonne, you need to get her to 45 loyalty. I know there's an option in her dialogue that allows her to hit 50 loyalty, but I don't know if you can also get the Yandere ending alongside it.
- Giving Vonne Charm or Stealth points is wasteful. Give them to Mallory and Rowan respectively.
- You can give points for Observation to Vonne or Rowan, but splitting between them is wasteful.
- At first glance, the best target for loyalty after Vonne is the sister you keep, but expelling the other sister for being the mole seems to lead to a drop in the one who stays, so that their loyalty becomes 25. This means they will not stick around when Talmage begins making threats.
- If you cannot make the surviving sister stay, you should block with her every turn in the first round Vs. Talmage.
So far, I've only made Thais the traitor (Althea is superior as a harasser and for having a generally more useful skill allocation than Thais). The first time I maxed Althea's loyalty before expelling Thais (which then reduced her to 25 loyalty, leading her to bail after Talmage's first round) and the second time I maxed Thais hoping that doing so would lower Althea's penalty when you expelled Thais...and it did: in the sense that Althea wasn't maxed so she lost less, but still wound up at the dreadful 25.
The issue is, giving points to Vonne for intimidation is less efficient than giving them to the sister you've kept.
Vonne taking over Mallory's Charm, and Rowan's Stealth and Observation is super handy because it means you have a super minion for Questioning, Stalking, Gossiping, Misdirecting, etc... but when the other sister bails Vs. Talmage in the second round, Vonne's low Intimidation score is going to hurt whenever Talmage makes a Charm attack. You are utterly reliant on items it seems, and I don't know if there's a way to circumvent the issue of going into the Talmage fight with someone who isn't going to bail on you and is good at Intimidation.