The original RE1 was my first RE game (although it was RE3 that originally caught my attention), and when I saw Chris and Jill, I expected them to be a couple, either right from the start or by the end of the game. Isn't it always like that? Almost every TV show, movie or game starring a male and a female lead ends up shipping them sooner or later. And those two were roughly the same age, they looked alright together, so I thought, why not? But when I played the game, I realised they didn't really spend much time together, and the little interaction they had didn't seem romantic to me. It became even less "romantic" in later games, so I agree with the brother/sister thing.
People in favour of Valenfield keep bringing up the reunion scene in RE5 where Chris cradles Jill in his arms, but that doesn't mean anything to me either because Jill was weak at the time and had just gone through hell, and it's Chris we're talking about, he just loves touching people and does it all the time. In the Remake, he gets close to Rebecca and grabs her shoulders every time he sees her (it gets ridiculous after a while), and also holds Richard's hand and strokes his cheek as if they were lovers, yet I never see anyone shipping Chris with either of them. It's clear that Jill means a lot to him and vice versa, and I can see why given how much they've been through, but a romance? Not so much.
I think the closest thing we have to an "official" couple in this series is LeonXAda, who kissed at the end of RE2's canon scenario. Sure, a kiss doesn't mean they're in a relationship, it also doesn't mean they're going to be together forever and marry and have children, but it does mean there's some kind of attraction, and I don't see that between Chris and Jill. But I guess the relationships in RE are like the stories of the Bible; you can read into them whatever you want and even find "proof" for your assumption if you just look hard enough.
