[Imperial guard, even Cadians, can't be allowed to reach the enemies first. Not when the Sisters of Battle have taken the field. The fact that the artillery barrage that just ended was a Tau missile battery ceasing fire so their own Kroot allies could advance, is immaterial. So is the fact that hovertanks are forming the spearhead, moving far faster than any imperial armor could hope to keep up with. (not that there are any tanks here, they're all at least a kilometer behind the trenches in vehicle parks.) That is what the Emperor provided melta weapons for, so charge in and use them! And if the Tau themselves use their heretical plasma guns to rip everyone apart from long range, then the Sisters will give them bolter fire and....probably die bravely.

One sister Superior however is leading her sisters in a way that shows that she had tactical acumen as well as faith in the Emperor's protection. Even as Tau plasma and railguns answer the Imperium's weapons, she holds the vital hinge between the Sister's charge and Mira's unit, preventing the two lines from becoming uncoupled as the power armor of the sisters allows them to outperform mortal men. In fact, it might be starting to look as though the main body of the Sisters is dangerously far forward.]

Conserve your ammunition! Take aimed shots, and let the Emperor guide them, do not make him grant each of you a miracle!