So, my bit of Theorycrafting (more just musings). I don't know much about the Inanna/Ereshkigal aspect. From my VtM experience, tho', and it's connection/inspiration/etc. to the game, I DO make some assumptions, although I fully expect there's not a 1-to-1 connection. I don't think any of this really qualifies as spoiler, so....
1. There's a connection between Sire and Childe (pretty obvious), and Sharon's particular history does easily explain her behavior to Laurie now.
yup!
2. In VtM, old, early generation vamps have a LOT of power. In VtM terms, PC could quite readily be, say, 5th generation and Sharon's a good ol' 12-13th. I suspect this has slowly been dawning on her (and rapidly of late). This is the easy stuff. One could start going down the rabbit hole and muse that Sharon turned Laurie before the PC inevitably would to have another hook on him. That'd be quite consistent with VtM, but I don't think so much in this story. A cynic might suggest that she already viewed Laurie as something of a rival, and if/when PC inevitably turned her himself, she'd be a rival with a much stronger generation.
Yes, Laurie was already a rival. Yes, if you flip back a few hundred pages, we established that the MC is 5th generation. Sharon is 10th-ish max. Sharon is furthermore Ventrue, Astrid is Toreador, the rest do not matter that much, except Eloise is also Ventrue as is Roland.
Sharon turned Laurie so the MC would not have to go through the pain and loss. Not to keep the MC from embracing her.
3. Calisto. Ahhhh, Calisto. My prevailing assumption is that she is quite aware of what's down in the cave and has been for awhile, and that's pretty much why she sired a childe - a fire-and-forget missile at dealing with a sticky problem. Hook him up with a damaged vamp (damaged in that Sharon seems to actually care) asw nursemaid, surround him with those who can teach him/unlock some of his power, hook him up with the Nos, who, if you're about to get in a scrum belowground, are the No. 1 clan to have at your side.
Calisto is not aware of what is down there.... yet. She is arrogant and assumes she knows more than she does. She is not infallible. She wants her childe to be the Achilles she deserves as a lover, or die trying.
There's almost zero risk on her part. If he succeeds, it will be because he's become quite formidable, which makes for a terrific ally (and one that is bound to her). She gets rid of a problem, she (importantly) doesn't let anyone else get rid of it, which could have meant the elevation of a potential rival, and she risks pretty much nothing. If he fails, well, she's better informed about what is evidently a more serious problem than she wished. Plus, in whatever level of vamp society in which she has peers, she gains a big Smiley sticker (with gold star!) for cleaning up that serious problem.
Calisto is like in a class by herself. The Goddesses are both so far beyond her that it is farther than comparing the MC to her. She is 4,500+ years old and 4th generation. The Goddesses are 10-15k years old and 1st-ish generation equivalent. Calisto is barely a blip on their radar for their upcoming fight. Only Virgil, Fabian, Roland, and the MC are worthy of note in the current vampire city, to be referenced with Calisto, but all are far below her. Virgil is probably an enemy. Could have been Calisto's first childe, but he is not. She said and was not lying, that the MC was her only living childe. Virgil is a Roman Vampire, probably descended from Ereshkigal's line, and is going to interfere with the rising Inanna to assist Ereshkigal, or interfere in the battle between Inanna and Ereshkigal, just as Calisto and her childe, and Fabian will be for Inanna, or both.
4. Last little bit related to Callisto securing a formidable ally in her Childe PC...he'll inevitably sire vampires in turn - whom also will be of quite elevated generation. Carmen, Ivy come to mind. Carmen, in particular, would (will!) make for a fantastic vamp. That's a nifty little power base being built up. She doesn't seem like she needs it, but...if this Inanna/Ereshkigal thing goes on, she very well might.
Yes, I have had the theory that Carmen would be the MC's childe, and she probably will be, but not in this novel. There is already one super powerful kid vampire and that is enough. She will become his thrall though.
MC will have a harem, vampire style- Sharon, if you do not piss her off too much somehow, Calisto, who will be your domme vamp mommy, Carmen, Ivy, and Laurie. Blood dolls added for flavor, and potential side girls becoming harem members if there is another volume in the story.
The one rule above all I remember about VtM, was when someone asked a very well informed source what the most common mistake players make was, "They think they can win."
(sorry if this is trodding over well-covered ground)
Yeah, this has all been covered pretty well, but that's cool.