Currently, Piglins will not attack players with an Identity that is either a Piglin or Brute:

if (EntityTags.PIGLIN_FRIENDLY.contains(identity.getType())) {
       cir.setReturnValue(false);
}

For other targeting tasks, the hostility component is checked to determine whether mobs should aggro on a suspicious player (hostile morphed player attacks hostile mobs). The issue with that in this situation is that Piglins will aggro on you for attacking non-Piglins like Creepers, which doesn't make much sense.

Questions:

• should Piglins aggro on players with a Piglin Identity that attack other hostiles?
• should Piglins attack players with a Piglin Identity that attack other Piglins?

should Piglins aggro on players with a Piglin Identity that attack other hostiles?

I'll vote against that, especially if you attack a Wither Skeleton or a Hoglin while disguised as a Piglin, since it wouldn't make sense for other Piglins to go after you just because you attacked their prey (Wither Skeletons and Hoglins).

should Piglins attack players with a Piglin Identity that attack other Piglins?

I would vote for this because it makes sense that when a Piglin attacks another Piglin, the other Piglin should fight back.

By the way, there could be a config option for both behaviors if you wanted.