Being older and more experienced than you, she probably can see that you are misguided and drawing conclusions based on superficial appearances.
Every Christian denomination 'stumbles,' (James 3:2), which is part of the process of how we learn and grow, (Ephesians 4:11-16). Joel Osteen and other 'prosperity preachers' have strengths which illustrate the nature of GOD while also 'stumbling' in the areas which you have obviously concluded make them 'false teachers,' (Biblical Greek: 'pseudodidaskalos').
"But false prophets also arose among the people, just as there will also be false teachers among you, who will secretly introduce destructive heresies, even denying the Master who bought them, bringing swift destruction upon themselves. Many will follow their sensuality, and because of them the way of the truth will be maligned; and in [their] greed they will exploit you with false words; their judgment from long ago is not idle, and their destruction is not asleep." (2 Peter 2:1-3).
The one trait common to actual 'false teachers' is a type of greed and appeal to licentiousness that swiftly destroys people and ultimately denies Christ Himself. 'Licentiousness,' (translated 'sensuality' above), is a false form of liberty offered by criminals which invites us to break the law, violate morals or abandon ethics. 'License' is the other false liberty based on government and law, offering us 'special' permissions over others based on favoritism. GOD's liberty is 'natural liberty' and contains neither of these, (2 Corinthians 3:17).
A genuine Christian teacher does not need to be 'perfect.' They only need to be guiding people to GOD's form of genuine liberty and away from the other two, thereby demonstrating a command of some aspect of Christ's teachings.
"Beloved, I pray that in all respects you may prosper and be in good health, just as your soul prospers." (3 John 1:2).
"And I say to you, make friends for yourselves by means of the wealth of unrighteousness, so that when it fails, they will receive you into the eternal dwellings. He who is faithful in a very little thing is faithful also in much; and he who is unrighteous in a very little thing is unrighteous also in much. Therefore if you have not been faithful in the use of unrighteous wealth, who will entrust the true riches to you?" (Luke 16:9-11).
"I have been young and now I am old,
Yet I have not seen the righteous forsaken
Or his descendants begging bread.
All day long he is gracious and lends,
And his descendants are a blessing.
Depart from evil and do good,
So you will abide forever. (Psalm 37:25-27).
"Jesus said, “Truly I say to you, there is no one who has left house or brothers or sisters or mother or father or children or farms, for My sake and for the gospel’s sake, but that he will receive a hundred times as much now in the present age, houses and brothers and sisters and mothers and children and farms, along with persecutions; and in the age to come, eternal life. But many who are first will be last, and the last, first.” (Mark 10:29-31).
Can you HONESTLY say that you are experiencing '100-fold prosperity with persecutions' after abandoning the old social models for family, business and agriculture and living according to Christ's new ones, (no longer based on the 'benefactors,' Luke 22:25, or their 'birth class' system)?
The righteous ARE commanded to be prosperous as part of our testimony about the wisdom and power of Christ. However, this prosperity doesn't come through government or criminal power. Many Christians have abandoned this entirely and believe that being 'poor' is how we become Christ-like, or they 'cheat' using law or lawlessness, justifying it by judging the 'rich.' There are powerful lessons of humility we learn by OVERCOMING poverty, yes and this does require experiencing it. These are important teachings of Christ also and perhaps you can teach others about them. But that doesn't give you or anyone else the right to judge those who are demonstrating the teachings which you are FAILING to demonstrate.
As the Body of Christ, we express the full righteousness of Christ only by bearing each other's burdens, (Galatians 6:2). That means admitting to both weaknesses and strengths, offering strength to others who lack it and willingly receiving help from those who have strength to give us to overcome our own weaknesses. The tricky part is avoiding 'idolatry' and pretending that any one of us is 'perfect' and has no weaknesses while judging others for theirs. That is the basis of 'false teaching' and we can always know it by its destructive 'fruit,' just as Jesus taught us to recognize it, (Matthew 7:16-20).
He also taught us not to judge others, or we would be judged with the exact measure and standard we apply to them, (Matthew 7:1-2). That is something I'm sure your grandmother understands and wants you to understand as well.
Faith is a legacy which can be nurtured and passed to the next generation.
"For I am mindful of the sincere faith within you, which first dwelt in your grandmother Lois and your mother Eunice, and I am sure that it is in you as well. For this reason I remind you to kindle afresh the gift of God which is in you through the laying on of my hands. For God has not given us a spirit of timidity, but of power and love and discipline." (2 Timothy 1:5-7).