One thing you could ask your mom is this: "If she was out in service, and found someone who belonged to a religion that taught everything the Bible says and followed it to the letter. However their religon had just one "wrong" teaching, would she tell that person to leave their religon and be a JW." When she says yes then ask "Even if the "wrong" teaching was just a small thing?" She will most likely will still say yes.
Then ask her "Would you drink a glass of water with only one small drop of poison in it?" Of course she will say no. Then the question is why is it ok for your religon to put "small drops of poison in the water and expect you to drink it.
Also study and understand the scriptures about Michael and the WT's ever changing teaching about who he is/was/will be. This is a purely scriptural discussion that can be used to plant seeds of truth about Jesus and seeds of doubt about the WT. It is a good starting point. With any conversation try and remember it is not weather you "win or lose" as you will rarely "win" in one conversation. Don't lose your temper or feel foolish if they seem to be "winning". That is what they are trained to do - Win at all costs, even if they distroy their own family and friends. They only care about looking superrior to every non-JW. Lood luck in future conversations and as has been mentioned she is your mother.