Complex sentence: Valenza is considered likely to have been a sister of John IV, rather than a daughter; in this case her parents would have been Alexios IV of Trebizond and Theodora Kantakouzene.
Split into simpler sentences: Valenza is considered likely to have been a sister of John IV, instead of a daughter. In this case her parents would be Alexios IV of Trebizond and Theodora Kantakouzene.