Alleria would join the alliance if anything, she fought with them in the second war, her husband is human and her only connection to the horde is Slyvanas who literally wanted to murder the other sister so she would also be an undead. But honestly Alleria and Turalyon are too busy with the Army of Light to ever bother returning to a faction.
On the PTR there is also a lengthy bit of conversation between Vereesa and Alleria about Slyvanas now leading the horde and how it basically sickens Alleria to think about it. https://youtu.be/eEHL7lKcmCM?t=1m2s