No, the relationship isn't forced.
Banri said "he must've gone through my phone" or something like that, to find out they're not related, so they were already going out when the BF found the secret.
Still, pretty stupid of the BF to think he actually had any leverage, and pretty stupid of Banri to think the same, I get she just didn't want the media to harass the brothers and sisters when they're already worried with other things but still.
"I thought about consulting dad" nah, I don't think you"thought"at all