You may or may not want to do what I did since it might not work in your situation, but I had a discussion about shunning someone who is unbaptized with my relatives. And I managed to get them to agree with my position.
The argument goes something like:
Do you think that people should have religious freedom?
Does not God want us to serve him of our own free volition?
(They should agree with this)
So am I free to make a decision about what I want to believe?
Same argument as previous question
"But if you don't choose God we wont have anything to do with you"
Am I truly free to choose to believe what I want if you are threatening me with the loss of my family? Would Jehovah want me to serve him because you are threatening to disown me?
I was brought up in this religion with no choice and if I am free to believe what I want, then I should get a free choice without any punishment.
My relatives agreed with this line of reasoning, so you could try it if you find yourself in a potential shunning situation.
However it might be more potent to just assume this is the case without a discussion and take the angle:
I have been thinking a lot about this and I have come to the decision that I can't prove that this is the truth to my self. Thank you for letting me have my own opinions and choose my own path.
Not sure which one is better or if either of them would work for you, I can only tell you what worked for me.
Another tip that might backfire but thankfully worked for me, is to have them ask the elders and hopefully they will tell your parents that they need not shun you. (there is no JW doctrine requiring them to do so)
My aunt went to the elders and asked if she could associate with me and they told her that it was fine as long as I don't try to unwitness to her, hehe :P (I had unwitnesses a little once in a while, so she was worried she might have to stop speaking with me)
However, this could backfire if they tell your parents not to have contact with you...
Best of luck regardless of how you choose to play it! :)