She is due to come back in September but I don't know how to play it. Do I send her a text and keep it cool, ignore the visit all together? Or say I'll go to wherever she is staying and be the bigger person?
What do you mean by "being the bigger person" here? What exactly are you trying to achieve, and for whose benefit?
I don't see how telling her you'll bend over backwards to be able to stand in her presence if only she'll let you is being the bigger person at all - it sounds desperate and like you have no dignity, under the circumstances.
My argument is that she can't be " the best Auntie ever" on social media and not know anything about my daughter or show up to see her.
Of course she can. She's proving that she can by doing exactly that. People can be hypocrites and liars. It happens all the time and it's perfectly legal.
Your options are to unfollow her, roll your eyes and ignore her, accept that this is just how she is, or the nuclear option: call her out on social media.
Personally, I would just unfollow her as you're not going to change her and you're deliberately torturing yourself by looking at what she's saying and doing with other people when you have the choice to just mute her and get on with your own life.