I would avoid debating or disputing any religious beliefs. All religious beliefs are based on faith and therefore cannot be debated in any logical fashion. If someone's faith calls for them to do to believe something, then they will do or believe it regardless of what information you have to offer.
Stick to secular behaviors she may engage in. Instead of saying that she is shunning you for being disfellowshipped, say that she is shunning you for having religious beliefs different from her own, and then point out how Jehover's Witnesses claim to respect everyone's right to religious freedom and even fight for this right in court. Use this to demonstrate the double standard of her wanting to have her right to worship as she pleases while at the same time punishing you for exercising the very same right.
Point out how she cherishes her right to raise her children in the faith of her choosing, without having to worry about someone of another faith trying to influence her children. Tell her that you also cherish this right and intend to raise your child the way you see fit. She would not like it if someone of another faith came along and asked if they could teach her children another religion would she? Of course not!
Tell her that Jehover?s Witnesses have won many court battles that ensure one's right to freedom of worship and you intend to use those rights to the fullest.