I am going to assume that she’s not merely being tactless in pointing out what are real departures between you and the teachings of the Church. IOW, I’m going to assume she …
OK, so she is being socially aggressive and accuses you of being in league with the devil because you are bold enough to tell her to quit abusing you. Is this new? If someone who never used to say things like this started to say them, I’d look for the cause. My two guesses would be a) some sort of reading that has prompted her to see the devil everywhere, b) a slip in mental health or c) all of the above. The very first signs of dementia in a woman I know was when she started becoming very moody and made accusations towards her family that were entirely out of character for her, not only in what upset her but in how upset she got and how she made her displeasure known. If this is a change in her behavior, keep an eye on her mental faculties.
Let us say that this is a long-standing habit, and you have simply had enough of it. You have no doubt found by experience that she’ll not endure correction on the obvious point that she’s accusing you of being in league with the devil because you’re correcting her lack of charity towards others. That makes this a “pearls before swine” situation. In that case, you draw a line: This is behavior we don’t tolerate, the boundary is not up for discussion, and this is what we’re going to do if you cross that boundary and will not apologize. Then do what you say you’re going to do. Follow the cardinal rule of parenting when you draw boundaries with people other than your children: Never threaten a consequence that you are not going to hand out, and never hand out the worst possible consequence first, not unless the offense is very bad and the offender WILL NOT back down. Leave something in your bag of consequences to up the penalty, in case some even worse infraction follows. Then do what you say you’re going to do.
Generally speaking, when someone at a large gathering commits a willful and substantial social insult, the victim looks for an apology. If none is forthcoming, the victim refuses to have anything more to do with the offender, and socializes instead with someone else. If worse follows or if the initial offense was sufficiently egregious, the victim and their entourage leaves, informing the hosts if the hosts were not themselves the offenders and didn’t witness the offense. (It is better to avoid a lot of fanfare when leaving. The idea is not to heighten the drama, but to walk out on it.) If the social insult becomes habitual, the victims simply refuse to socialize with the offender until some contrition or at least some firm purpose of amendment is offered. If someone at your gathering insults a guest and you find out about it, it is your job to ask the offender to either apologize or to leave, and to encourage the victim to stay, instead. (If a member of the household is the offender, they are sometimes banished to their private rooms, of course.)
If this means ushering your own mother out of your own home, so be it. You have the duty to protect those under your roof from social abuse, including yourselves. Excepting someone who is tolerated because they don’t have all their faculties, no one gets a free pass on that.
"…I’m not this bold…