The event is indeed well written, with proper buildup and have a lot of renders put on it, i will give you that. In terms of quality is probably one of the best ones in the game yet.
But why Ellie was without a bra and caring so little about being stared at with such carnal desire by Dylan (someone she is disgusted of)? It's the same Ellie as the one we've met on the game, but she acted really different from how she acts with Dylan normally.
IIRC this was after the whole Viagra at the beach prank that she played on Dylan. It may be that what Sophia said to Ellie, telling her off before they got out of the car, maybe it sank in. And maybe she felt guilty that she had overstepped the boundaries of sibling rivalry.
And then she needed Dylan's help so she had to tone down her usual insults to Dylan.
And then finally, Dylan probably surprised her when he said he just wanted a hug as payment. In most incest games when the female LI asks for a favour, usually the payment would go beyond a hug.