so luca brasi died from the get go. one of the first dead when the gang war started. what if rather than luca brasi dying he survived. then say that don vito died of natural causes(which he did) would luca most likely be loyal to his kids? why was luca so loyal to don vito anyhow?

There's almost an entire novel chapter that answers that question.

Here's a thread I made some time ago on that subject: Luca Brasi - alternate timeline

If Luca survives, it changes the arc of the novel and film. Consider this: what if Vito dies right after being shot? That changes the arc also. Everything that happens in a novel or film is prologue. There's any directions in which a novel or film can go depending on prologue.

There are two possibilities with Luca's survival, hinging on one question; does he reflect and feel he was put in an unnecessarily dangerous situation when he met with the Turk?

If so, he will find himself in quite the quandry. He can no longer trust Vito (and, by extension, his family) but Vito's enemies tried to kill him. He would have nowhere to turn and would either flee the country altogether or go out in a brash attempt at revenge.

If not (which I think would be the case, as I doubt his mind would allow him to contemplate the Vito would do such a thing), then he remains loyal to Vito, and by extension, his sons. If the meeting didn't cost Vito Luca's loyalty, nothing would. While Michael would probably phase him out upon taking over, he would still provide him with a role and treat him with great respect.

If Luca survived, I don't envision his loyalty to Vito wavering at all. If anything, his monumental rage would be directed at Sollozzo and the Tattaglia's who did try to murder him. Michael would have continued to utilize his services and Luca wold have responded to that use. Like Tom, through Michael, Luca would have continued his loyalty to Vito.

One interesting wrinkle would occur when Neri joined the family and eventually became a special jobs guy. This is where Luca could begin to feel left behind.

Pete, I don't see any potential conflict. As the novel explains it, Luca chose to be commanded by Vito regardless of any other variables. I don't think he would have been concerned about Michael's adoption of Neri nor about any ranking in Corleone family.

Luca was Vito's man to the death--Vito's or his. Sol knew that, which is why he waited until he killed Luca before trying to kill Vito. If Luca had survived, he'd have moved heaven and earth to avenge Vito. As Hagen told Sol: "Not even Sonny will be able to call off Luca."

If Vito had died of natural causes, it's a question if he'd have transferred his loyalty to Sonny or Michael. Luca was his own man, he had his own reasons (as the novel points out) to be loyal to Vito, and as we saw in the film, Vito saw him alone, without Hagen as intermediary. On the other hand, the Corleones were his only family, and provided him with a rich living.