It's Treason either way. Winters is ranked above three Senators of the Empire, that should make her a target, not an ally.
Plenty of Aisling followers have been spotted, winged with Hudson and Winters players, in Empire headquarter systems, murdering Empire aligned CMDRs.
That doesn't follow. From a narrative point of view, the principle opponent of all the Imperial factions are
each other, as they're all competing to be in a position to secure the throne for themselves once the Emperor dies. As I understand it, that's the justification for there being so many Imperial factions; the Empire is about to implode into a dynastic conflict as multiple pretenders to the throne try to take the fancy hat for themselves. Yeah, the Emperor is claimed to be alive and well, but it's pretty clear that even if he's making a recovery from his current illness (and I note that there's been nothing on GalNet about him making a public appearance...), it's pretty clear that he's on the way out in the relatively near future.
In that context, it makes
perfect sense for them to be making arrangements with outside powers. Pretty much every dynastic conflict in history had every side involved attempting to get outside backing to legitimise their claim. In this case, you could imagine that Aisling (or her backers) are thinking in terms of Winters winning the next election in the Federation (even if that means Hudson has to suffer an 'unfortunate accident'), with an agreement that once that happens, Aisling would be recognised by the Federation as the official inheritor. The Federal Navy helping her to 'restore order' to areas of the Empire 'controlled by rebels and illegitimate pretenders' would probably guarantee her victory, after all.
I suppose it technically is treason, but only if Aisling fails to claim the throne. That's no great loss, because chances are that once the next Emperor or Empress is secure on their throne, their erstwhile competitors are getting purged or exiled
anyway. If she wins, on the other hand, well, winners aren't guilty of treason on account of them being the ones that get to define what treason is.
From an in-game PoV, that basically justifies any Imperial faction getting chummy with any non-Imperial faction. Their relationship with each other, on the other hand, should range from hostility to the sort of uneasy cooperation you get when both sides
know the other one is plotting to stab them in the back at some point. They certainly shouldn't really be friendly with each other.
Unfortunately, the game mechanics actively disincentivise powers from the same major faction fighting each other, so the 'narrative' isn't really reflected in gamplay all that much.