Complex sentence: His inheritance was held in trust until he became of age, but the funds released by the trustees during his minority were insufficient for his education, so his mother (successfully) petitioned the courts for the release of additional money.
Split into simpler sentences: His inheritance was held in trust until he became of age. The funds released by the trustees during his minority were insufficient for his education, so his mother (successfully) petitioned the courts for the release of additional money.