How are your older parents dealing with this change?
My parents actually seem like they toned the preaching down as they grew old. I think that in their case they realized all the damage that they created in the family, and how much they alienated people around them due to their preaching and proselytizing. Also, they loved to argue, so they were never in agreement as to when/where was appropriate for one of them to preach.
You shouldn't feel embarrassed by your mother. Let it be. That's how I came to terms with my parents' behavior.