Maybe they were very young back then. Or they are so used to their new lives that they have forgot.
WAHAHAHAHAHA.
at all the crazy choices those goddesses are making; can't blame them for rejecting the war, but not keeping tabs on the events, participants or trying to gather info on what had originally happened makes them look thoroughly ignorant...and that while they've been implied to be "cooking" something big in the main altar in Istar, not recognising a fellow goddess' soul and one of them repeating the blunder of falling for one of their greatest "enemies" as fast as blinking. 
Or have they been referred to as 1800+ years old?
I assume their original bodies were 1800 years old, because nothing else seems to add up.There's one thing that gets me really confused: what "battlefield" were they trying to escape while making the possession deal if the twins have been stated to be around 1800 years old and the ancient holy war happened 3000 years before current events?!
I don't think I mentioned it on here but I've been thinking for a while that the whole reincarnation thing might work like with a phoenix in a sense. Once Elizabeth's current incarnation dies the next one is born (not entirely sure how exactly it'd work tho)
Does that mean the twins' souls are still in the bodies?! I was under the impression they gave up their bodies as they were already dying and their souls moved on, while leaving the "shells" for the goddesses to use. And the phrasing still makes it odd, since it sounds as though the same 2 goddesses who were trying to escape the unknown war(it doesn't sound as it was about the sisters, even if they were said to have gone through war and starvation) weren't actually "hiding".I assume their original bodies were 1800 years old, because nothing else seems to add up.
The "phoenix" technique is more of what I have in mind as well; it seems as though a lot of her original power was either used up in the Seal, or turned dormant for some reason(Meliodas claims she has power that needs "awakening", plus I doubt that in the fight against Hendricksen she only had a 1925 PL, she did dispel "Dark Nebula" and aoe healed most of the Liones population) and with the little she was left she could only "recreate" a human form for herself. I'm still waiting for the animation of the memory "trip" Zarathras offered, because I had the odd impression the blast in Danafor wasn't just made of Hellblaze but it also showcased her light wings, there's something in the midst of the flare that resembles the inner curves of her spread out superior pair of wings.
I doubt the twins are still alive - seeing as there's nobody to "hide" from in the current era, I feel like Jenna and Zaneli would have already left the bodies of the twins; the phrasing makes it seem, just like you said, like the twins gave them permission to enter the bodies and then passed away, ooor the twins' souls are alive, but their lives depend on Jenna and Zaneli.
Not all elements from these episodes can be considered canon so maybe it's not destroyed.
Does the horn make a chronological appearance in any chapter, canon or omake, after they left for Camelot? I can't recall seeing it or even being mentioned. And I thought that the scene where he's mentioning the vessels happens in the omakes about the mission right before they were staged for Zarathras' murder and chased out of the kingdom.
I don't remember it appearing either.
It's not in the manga. Meliodas destroyed it in the first episode of Signs of Holy War with Hellblaze. Certain things like Vivian's ring seem to be canon and we have not see the horn since that discussion with Ban so some think that it's really been destroyed in the manga.