Complex sentence: Recent excavations suggest that this was in fact the second Christian building on the site, and may be some decades later than traditionally thought, and built as a mausoleum for Constantina's sister Helena in the reign of her husband Julian the Apostate.
Split into simpler sentences: Recent excavations suggest that this was in fact the second Christian building on the site, and may have been built decades later than traditionally thought. Recent evidence shows that it was built as a mausoleum for Constantina's sister, Helena in the reign of her husband Julian the Apostate.