I'm trying to understand the below. It's Orberg's adapted Livy from LLPSI Cap. XLI.
Nondum maturus imperio Ascanius, Aeneae filius, erat; tamen id imperium ei ad puberem aetatem incolume mansit. Interim Lavinia pro puero regnavit. (Incertum est - quis enim rem tam veterem pro certo [= certo (adv.)] affirmet? - hicine fuerit Laviniae filius an filius ille maior, Creusa matre natus, Ascanius sive Iulus appellatus, quem gens Iulia auctorem nominis sui esse dicit.)
I don't usually translate but I want to be sure I've got this right:
Not yet ready to be emperor Ascaius was Aeneas' son; despite the purple having been reserved for him since the time of his youth [ad puberem aetatem]. During that time Lavinia reigned on his behalf. (It's not certain - who could confirm such an ancient thing - whether he was the son of Lavinia or the older brother, son of Creusa, Ascanius, also called Iulus, [and here's where I am unsure] who the Iulian tribe say is the beginning of its name...
What I'm really not sure about is the construction (subject - object relationship) of this last part. 'Ascanius sive....dicit'