Complex sentence: Zenobia held the reins of power in the kingdom, though she never claimed to rule in her own right and acted formally as a regent for her son; the latter was kept in his mother's shadow, never exercising real power.
Split into simpler sentences: Zenobia held the reins of power in the kingdom, although she never claimed to rule in her own right and acted as a regent for her son. Vaballathus was kept in his mother's shadow, never exercising real power.