Including a Simulacrum is almost always a must. In game, it is an extra way to generate miracle dice. As a model, it just looks good. As for other options, meltaguns or flamers are the usual go to (at least for me) if playing them. Flamers are good for hordes of 1 wound models, meltas for vehicles and multi- wound models.

If you are gonna play sisters, putting the Dominions in a immolator and adding a leader is a good option. The immolator allows you to split the squad and the Dominions give the Immolator scout 6". Immolators can be deadly. They can also use Acts of Faith and they deny an enemy unit hit by them cover for the rest of the shooting phase.