Hi booker,
The main problem you have is not finding evidence to refute what she says, that?s the easy bit; the hard bit is getting her to accept that evidence. For example, if the WT prints an article stating that the Moon is made of green cheese then all JW's will accept it as fact because the article came from the faithful and discreet slave, who are guided by Holy Spirit. Even is you take them to NASA headquarters and show them moon dust and a piece of moon rock they will use that same old argument, " that moon dust was created by Satan to deceive mankind", an argument often used by the org when archaeological discoveries have been made that prove one of there teachings wrong.
The only way you will win the argument with your mom is to get her to prove herself wrong or get her to contradict herself on this point, use there own arguments against them ( the benefits of pioneer training school ), Use the argument that they use against Mormons.
Also start off with a leading question. Ask her. Are all the beliefs of the Society based on what is printed in the Bible or are any of their beliefs based on an opinion or a man made assumption ? You already know what she's going to say, but get her to say it, it's important for your argument later.
When faced with a Mormon, the witnesses argue that Joseph Smith could not have been a Prophet of God; the argument they use is that the Bible, in Deuteronomy chapters 13 and 18, provides two tests for anyone claiming to be a prophet and speak for God. If the person fails either test given by God, then we can know for sure they are a false prophet and we are to reject their teaching.
The first test is in Deut, 13:1-2, their argument against Joseph Smith is that where it states that if a prophet gives a portent and it comes true but then tells you to follow him he is a false prophet.
Now the second test is the cruncher you can use.
It's in the same verse that your mom quoted Deut 18:20-22, their argument against Joseph Smith being a prophet is that in this scripture the God of Israel tells his people how they can keep from being deceived by someone claiming to be a prophet and that is if even one of the prophet?s sayings do not come true he is a false prophet, first part of verse: 22, When the prophet speaks in the name of Jehovah and the word does not occur or come true, that is the word that Jehovah did not speak, ( in other words what he said was made up )
They say Joseph Smith is false because what he said would happen did not occur, ( there are 7 documented events he predicted that did not occur at all or within the time he said they would ) .
The Society has stated events that would occur that have not, and made predictions that didn't come true. What?s the difference ?
Your mom argues that so did Peter but he said, "thus saith the lord" in his error. Ask you mom to show you where it says in the Scriptures that Peter must have said "thus saith the lord" in his error, or are they assuming he said it in error, and remind her of the question you asked earlier, Are all the beliefs of the Society based on what is printed in the Bible or are any of there beliefs based on an opinion or a man made assumption ?
Ask her to tell you which of the following is true. Either Peter was a false prophet, there is a contradiction in the Scripture here as to how you know when a prophet is false, or the society is using Deut 18:22 one way when arguing against the Mormons and another way when applying it to themselves.
It's just one example of how the org twist scriptures different ways to suit the occasion.
You can try, but I fear 40 yrs of programming will prevent her from questioning the org, the only way to find fault with the org is to look for it , an she's not looking, she's defending them.
Keep us informed.