Its a tough one and deleting her would definitely take some courage.
I still remember my psychotherapist talking to me about doing things to protect yourself and if others dont like it, then that's for them to deal with - its their issue. I also had a tendency to build a simple problem up until it became a huge thing to deal with. My mind would be rampant with various scenarios and at the end of the day - nothing I had been stressing about would happen anyway. I am sure your Aunt wont even notice or wont even make much of an issue about it.
My DD deleted me from her facebook, when I eventually found out, I asked her about it and she said that she wanted some privacy as she is 19yrs and it would be like me reading her diary. Fair enough and good on her!
You could try deleting your Aunt in stages, maybe from you DD's page first. If your Aunt says anything, just say it must be a mistake and your DD wouldn't have done it on purpose. but then dont 'add' her again.
As I said, my AH hasn't even noticed that I have deleted him from my page and I am his wife.
Doesn't this all sound like a 'mess' to you now, that you are worrying and stressing about hurting your Aunts feelings, whilst you/DD/SIL are all dealing with the consequences, time to detach!