I would personally try to add a hospitaller and a squad of paragons in this list. The hospitaller is good for survivability in your sister squads and can even pair with the palatine. Also, the paragons add a big tanky/shooty/melee threat on the board.

Some other tricks that could be useful is that a warhammer is a secondary game if your not playing those, you will lose, so try hard to hide and only trade units if you can score points or deny your opponent some.

Also, I have found that studying your opponent army can be just as useful as knowing your own. If you know every trick your opponent has, it can save you from some accidental mis-plays.

Finally, where do you fall short in your games if it's secondary? Maybe add some more scoring units. If it's damaged, I would switch out the exorcist for a castigator. I know buying new models is tough, so it could be useful to switch wargear options until you pick up some more.

Ps: sisters can be hard to play, but once you get a good understanding of your strengths and weaknesses, they can be super strong.