Vito Corleone did not fear Santino...nor did he fear being 'ratted on' by Santino.

And even if Sonny were capable of turning over his father...which he was not...how exactly would he do this? Run to police headquarters and explain how a few years back when he was just a kid he followed his pop around and watched him kill a guy?

Due to among other things his father's connections with the police, judges and local politicians...the likely reaction would be a pat on the head and a suggestion to run along home to the mall.

Vito had no such fear; that is NOT the reason he backed down.

Apple