So the scenario you have is that two characters have Robilar's, but one has Sidestep, so the following situation would go:
Character A makes a normal attack against Character B (hereafter just A and B for brevity). B can make an AoO once A's attack is resolved. At this point, A could make its own AoO, and B could use Sidestep.
Is that the scenario you're describing? If so, there pretty much won't be official rules on that because it's such an esoteric rules interaction. The thing I think would be most reasonable is the character with Sidestep could use the 5FS before the opponent's AoO because the character is prepping for such a thing thanks to the training of having those feats in the first place. One might envision making the AoO and taking the 5FS as a fluid movement of an attack going into the 5FS.
Incidentally, a 20th level character can't take epic feats. As the epic feat rules state:
"At 21st level, and every three levels thereafter, the character may select an epic feat in place of a nonepic feat."
After all, a character isn't considered epic yet until it has 21 or more character levels.
Right, there is one character who is 21, three who are 20 and one who is 19 (a cleric who doesn't mind spending XP for spells).
Yeah, that is generally the situation.
Characters A and B both have Robilar's and Character B has Sidestep.
A attacks B, generating an AoO.
B attacks A, triggering A's Robilar's and his Sidestep.
Does his Stepstep allow him to 5FS before A's AoO? That is what I am trying to decide.