James: Contrary to some James hater’s claim that he was “saving his own skin”, James would not have gotten in trouble at all, because he WAS NOT involved in the prank. So no matter what happened, he would not have faced any consequences.
Sirius: If Snape was dead, he wouldn’t have been able tell anyone of Sirius’ role. The only way Sirius would have gotten into trouble is if James or Remus ratted Sirius out. I don’t think James and Remus would have ratted him out. Hence, he might not have gotten in trouble either.
But there is a small chance that their guilty conscience might have made them tell on Sirius or make Sirius surrender (But a very small chance). There’s also a chance that Sirius himself grows a guilty conscience and confesses up, especially if Remus got in trouble. In this case, Sirius could have faced repercussions like expulsion and Azkaban. But then again, he didn’t commit direct murder. It’s more of a case of negligent homicide. Sirius was still underage too, so I’m not sure of how much repercussions he would face if his role in the prank got out. If Lily said that Snape had suspected Lupin was a werewolf prior to going down there, there might be even lesser repercussions on Sirius.
Another repercussion is James and Remus possibly cutting off from Sirius in anger, though they might have patched up with him eventually (maybe only after years).
Remus: Poor Remus would have felt so guilty all his life. He would probably have been expelled. At worse, he could have been executed. Sirius confessing his role in the deed would do little to get Remus off trouble.
But then again, James and Sirius might have tried to hide his body to avoid getting Remus in trouble and try to frame the crime as something else. Maybe Dumbledore might have covered up for Sirius and Remus.
James and Lily’s future would depend on the circumstances. Maybe she wouldn’t, since he really wasn’t involved. But if she knew Sirius was involved, she would not associate with James unless he cut off from Sirius. So this might be a conflict for James if he still wanted Sirius in his life.
If things were still aligned for James and Lily to get together and have Harry, they would not have had to hide from Voldemort with no Snape to reveal the prophesy. But the fate of the wizarding world with Voldy still being in power would have been at stake.