What if Sarina had been a grown woman (maybe working as a doctor or nurse in the hospital), and Goro had been a cute little boy (perhaps her patient, or maybe they knew each other some other way)? I think, post-reincarnation, Ruby would be far more touchy-feely with Aqua than she is canon even. I mean, we already see that being twins isn't going to stop her crushing on him once she finds out his identity in their past lives. If she fell for little!Goro, I bet she'd fall for Aqua just as quickly. >:)
They'd probably be more closely matched as they were growing up than they are in canon. Ruby has the advantage of having already lived as an adult, but Goro was a pretty smart kid, so he probably wouldn't be too far behind in terms of mental development. Not as much as precocious little canon Aqua, but others would still find both of them very clever for their age.
Anyway, since she feels like she doesn't need to hide (with their being no risk of Aqua trying to boss her around and act like her senior), it's quite possible that she'd reveal her identity to him earlier, with him of course revealing his in turn. I think this would definitely affect their relationship, since if they found each other prior to Ai's passing, Aqua wouldn't have all that trauma to deal with, and he'd likely be less inhibited about getting close with her.
As for how Aqua would feel about it... Although he'd be less restrained, the fact that Ai is still around might also give him pause. After all, he's lucky enough to have been reborn as his favourite idol's son. He doesn't want to jeopardise it at this point by doing something silly, like fooling around with his sister. But Ruby is very insistent. She keeps telling him that it's fine, that they just need to be a little careful, that so long as nobody is watching too closely, they can have all the fun they want. She's older, so she knows better, and anything she tells him has to be true. Right?