I'll give you a weird (but very plausible theory). I say that Greg Scarpa Sr. himself whacked out his brother Sal.

How do you like them apples?

Not that I have even one shred of evidence to confirm my thought. But knowing the depraved individual that Scarpa was. And the fact that he was hyper-vigilant about anyone finding out that he was a rat, or spreading that rumor to others.

My theory is that Sal discovered his brother was an informant. Greg was fearful that Sal would tell the Colombo administration his belief. So Greg killed his brother to silence him.

Because after Sal was murdered. Did you see any retribution from Greg over his beloved brothers death? NO. Did he even make any major attempt to find out who did it? NO.

So there ya go!