Don't argue the details of what she says. Instead, focus on the behavior. If she oversteps, use calm, matter-of-fact scripts like, "I'd rather not discuss that topic".
That phrase appears to be a misheard or slightly altered version of a lyric from the song by Ernie K-Doe , released in 1961. The actual lyric is: "Sent from down below... mother-in-law, mother-in-law." mother in law bends my will better