It's important to note that this is also the book where they directly bring up that the two aren't talking, back when Mab uses Fix as a proxy to inform Titania because "She's not taking my calls".She doesn't seem to actually have anything against her sister so... why isn't Titania taking Mab's calls? No, my money is on something happening back in Hastings that meant they were not allowed to see one another. And that having her show up to help fight Ethniu will have Consequences.Consequences that will, naturally, be all Harry's fault.
An interesting notion! I like it quite a bit. Perhaps Titania is upset with Mab choosing to do something that means Titania will have to end Mab? I agree though that the two of them speaking again was just as likely the milestone as anything else in the last two books.