Why did police arrest only three?

Let me tell you an intriguing puzzle that will surely make you scratch your head. During the complete lockdown, when everyone was strictly instructed to stay indoors, two sons and two fathers were out on a bike ride without any valid reason. It was a clear violation of the lockdown rules, and the police stopped them immediately.

However, even after checking their identities and the bike’s registration, the police arrested only three of them and let one person go. Can you figure out who was the lucky one who escaped the arrest?

If you think about it, the answer is not as simple as it seems. It’s not about the bike’s registration or any identity mismatch. The trick lies in the relationship between the two fathers and two sons.