The combat potential of the daughters of the earth is much higher due to their higher stats and powerful spell strike.

There is no significant imbalance in this pairing. At least, this is indirectly confirmed by the fact that opinions are divided.
Perhaps the daughters of the sky need a slight buff (or a slight nerf for the earth daughters), but there's no need to make them unconditionally powerful spellcasters with an initiative of 12.

At level 4, there are already plenty of damage dealers. They often guard key passages. The probability of imbalance will be even higher: some will have bone dragons or vampires guarding the passage, while others will have daughters of the sky, and what should they do?