Gregorius21778 said:

I'd rather boot them out, rather than ruin it for the other players who want you to GM, but yea, I mostly agree with this.

If one player is completely unreasonable, you can normally make this known as a group and turn them around. If not, there are harsher solutions (let's not start with an exterminatus though), but as I take it these are not an option here, as she is the mistress of the House of Gaming.

This quite clearly means taking the paths of Slaanesh or Tzeentch to get her in line with the party. She obviously has a slightly skewed perception of chaos, so that is the first thing to remedy. In theory, anyone could fall to chaos, including Sisters of Battle (very unlikely, but not impossible). Let us be honest, the right hand man of the God-Emperor himself, the Warmaster Horus, more honoured then any of the God-Emperors loyal servants, has fallen to its temptations. Many of the Primarchs, the ideal creations of the God-Emperor have fallen to chaos. The Emperor himself is not insensitve to the soft lure now his mind starts to fracture, even if he till now has withstood it. So, a sister of battle, however virtuous she might have been, can fall, and can fall very deep indeed. It would even make a very interesting character if you ask me.

Next step is finding out what she likes. There is indeed the danger that she thinks Black Crusade is just about playing a slobbering berserker. I think we do FFG and the Warhammer Universe injustice if we would play it that way (which does not mean you cannot if you wish to, but to me that would be booooring from the first session on). Most players have 'archetype' preferred characters. In my group, you can be just as sure one of the players will wish to swing and sword (or powerfist) and get himself shiny armour as you can be of me diving headlong in forbidden lore and playing a more intrigue/magic focused character. If you can indentify her preffered style of play, see if you can tempt her with character ideas that would fit her and the party.

And if all else fails, use chocolate!

                                                           Friedrich van Riebeeck, Navigator Primus, Heart of the Void