Well, I have no choice but complain about the believability, if that's how you put it. Because it's basically zero - this engagement has been going on for years and the girl, while still a child, is not young enough to be clueless about what that entails. Unless she's delusional to the point of rejecting reality, it's simply not possible for her not to know. Also, why would she insist on tagging along, to the big sister's chargin, if she already didn't suspect what was up?
To be specific, he was more concerned that intervening would mean that the conditions for saving the kingdom (i.e. kill the unkillable monsters) would not be met. The game stated that what was needed was the "power of love" between the heroine and the love interests. This turned out to be false, but it's understandable why he'd seriously hesitate, even when he was already convinced the guys in questionj were no good. No one gets to be happy if everyone dies, after all.
Yes, it's the wrong thing to do if the only reason is some personal gain for you. However, I think the stakes need to be considered and these two stories are a nice contrast. As I've said before, in Mobuseka it's a question of death or a miserable life for Angie (and she wasn't directly tied to the final battle in any way), while in Olivia's case the plot was already derailed by a third party, so leaving her alone would likely have worse consequences than acting. In this story you have the option of helping a teenager wrestle with her emotions, which might be nice, but you already know that even if you don't do it, she'll be fine shortly. Said teenager is a key part of the plot, like Olivia was, so there's no telling what will happen if she grows attached to you now. What is certain is that your plan to "be a mob" will be about as realistic as Leon's - you're choosing to get deeply involved.