Why can't my sisters heal?

They can only use the ability twice per battle. You should see a little number over the skill that tells how many casts you have left for the current battle.

In addition, if your sisters get an extra action due to good morale, they will not be able to use their ability during the extra action. Attacks/movement during the extra action is only 50% of what it would normally be, and you are not allowed to use skills. You can however use the heal as their normal action, and then use the extra action to attack/move as normal.

They can only use their heal twice per combat. (And as you said once per round) If you look at the icon there is a number which indicates how many uses you have left.