"Thank you for the invitation. My conscience will not allow me to set foot in a Kingdom Hall. I am willing to discuss the reasons with you if you like, but be aware that such a discussion may be damaging to your faith. I will leave it up to you to initiate the conversation if you wish."
I think that's a little formal and apostate-sounding; I've no intention of de-converting her, I'm happy for her to live her life if she'll let me live mine. I don't want to come across as haughty or superior. I may just have to say that I'm busy on this occasion, as anything else will likely sound too stand-offish and could be damaging to familial relations. This is the first time she's asked since I left so I doubt it'll be happening too often.
We have also had invitations. If you do not explain to them why you won't attend, they will just continue to invite you. If you feel comfortable, speak from your heart. Be honest.
We have had to do this with our family. We told them we love them and nothing will change that. We will be there for them BUT if you don't believe in the doctrines, tell them.
You have one advantage, you are not baptized.
If you dodge them, they will just come back to you with more invites.
If people WANT to be in your life, they will, If not, at least you were honest with them. Good luck. Let us know how it goes.
It's frustrating because I have had these conversations with my family, albeit not for many years now. It just doesn't seem to go in, they always believe it's possible that I may come back. It makes everything smoother if the topic just never comes up. It's difficult to know how to approach this; I may use the busy card this time and see how long it is till I receive another invitation.