I sometimes point out to my screenwriting students that the opposite of love isn't hate it's indifference. As long as two people are fighting, they're still emotionally engaged with each other. Therefore, we're not surprised when, in a romantic comedy, the man and the woman who've fought all along suddenly fall in love. In fact, we almost expect this.
In the game example given earlier, the fact that the sisters disagree sometimes makes us more likely to believe they're emotionally involved with one another. It's a sign of Chemistry.
Perhaps one of the best examples of this technique was in the first Star Trek series. Bones and Spock would continuously make jabs at one another, which heightened our feeling that they belonged together. And who can forget the back and forth jabs between Han Solo and Princess Leia?
In the cases of Spock and Bones, as well as Solo and Leia, they wouldn't just fight with each other, but they'd also risk their lives for each other in a heartbeat lest anyone wonder if there wasn't real Chemistry there.
Thus, Chemistry can be more complex than just having two people fight or just having them be friendly. The conflicting feelings can derive because the characters feel different layers of emotion toward each other (described in Chapter 2.8, "NPC Toward NPC Relationship Deepening Techniques") or because one or both of the characters feel actual affection for the other but covers up this affection by pretending to be antagonistic. (Covering up emotions is an NPC Deepening Technique. See Chapter 2.2.)
These kind of approaches are more sophisticated, but that doesn't mean they're always the better choices. They're simply additional options for creating emotional engagement on behalf of the player.