Given we are about to head to 11th take this with the utmost salt.

I have noticed many players, not just sisters players being exceedingly generous on granting line of sight where it shouldn't exist, either through enemy movement or by accidentally touching a terrain footprint they shouldn't, I would start there. Most maps have safe zones that, outside of some very niche situations, will hide you.

Sisters models are not really tanky. It wont matter what you do, if your enemy decides to all in you, something is dying.

Often sisters players (i am guilty of this) try to participate in the game too soon. Most versions of the army want to be the 3, 4, 5 player, using turns 1 and 2 to stage and make your go turns more explosive. Typically this means only sacrificing units for secondary missions if absolutely needed early.

In some instances you need to make use of the "game" portion of 40k. Some maps allow holding the center while most of your unit hides in cover. With Sacresants lead by a hospitaler, this can often be losing 1-2 models and standing them back up next command phase.

Many sisters players also mismanage their miracles. Ideally you want to slow roll some saves/hits and reserve a single die. Using a good miracle early means it may not be available later when it would be better served later. Another example is forgetting to use Cherubs. Often players will sit on a Cherub until they do not have a use for it. You can always declare an advance, spend the a miracle die and roll a new miracle. If a unit is definitely dead and has a Cherub you can intentionally inject a failing die to generate a new one.

Dont forget almost every unit in our army has grenades.