Complex sentence: There was an added incentive for mothers to achieve freedom, as only children born of slave mothers would be considered slaves under Spanish law and children would be much more likely to pay for the freedom of their mother than that of their father.
Split into simpler sentences: There was an added incentive for mothers to achieve freedom, as only children born of slave mothers would be considered slaves under Spanish law. Additionally, children would be much more likely to pay for the freedom of their mother than that of their father.