My automatic reaction would be to go no contact. Nobody needs that bigotry. Would her not being in your lives really matter? Might she ever be seeing you more frequently and influencing your DDs? (Though perhaps they're at an age to understand Auntie Flo's got weird views, which no-one else in the family agrees with?). Does she spout her bile at every opportunity IRL, or just from the comfort of her keyboard, on her own FB?
However, your situation's made more difficult if you have to maintain (even minimal) civil contact ahead of the visit.
Will you actually be staying with her, or is it more of a fleeting 1hr 'duty call', for the sake of your DH's family ties? The first scenario is tricky, as online fireworks beforehand would make for in-person awkwardness for you, whilst she called the shots in her own home.
The second's easier and could be left to DH & the kids to do alone, surely. Migraine to absent you, if necessary.
Depending on the above, and your judgment call as to potential consequences, I guess one option would be:
A succinct, private message:
"I find your racist posts offensive. I've explained many times my reasons for strongly disagreeing with your views, to no response. I'm struggling to reconcile them with the person we know, who's always so kind and thoughtful to our DDs. I am therefore choosing to leave/ block/ unfriend."
Ball's in her court then...
Gwenci makes an excellent point: "...he spouts forth some frankly abhorrent views. I call him out every time. It gets me nowhere and I'll never change his mind, but I do it anyway because I never want him to think for one second that I'm in tacit agreement with him."
Well done for challenging! Good luck.