Lets get this clear. This is a case of elder abuse and elder abuse is a crime.
I accept that this person has mental issues - but elder abuse is a crime, whether the person has mental issues or not. All that is different is how the police, justice and mental authourities react to it.
This person is behaving in the classic abuser manner, first Mr/Mrs Nice, then Mr/Mrs Nasty, then Mr/Mrs Nice, and so on ad infinitum. Winding you up and keeping you there all the time worrying and wondering what they will do next, meanwhile watching you like a live fly stuck down with a pin, wriggling around and tryingto escape, but unable to do so. You have a choice. Either you keep suffering or you take some action, there is no alternative solution.
So what do you do? You prepare your dossier of as many of the messages you can get together as possible, along with a timeline showing how long it has been going on and the frequency of the messages. Write about the mix of nice and nasty. Everything.
When you have completed your dossier, go to the police, tell them that you are being psychologically abused by a memeber of your family and that you consider yourself a victim of elder abuse. You could warn the perpetrator in advance that you will do this if they do not stop, but if having made the threat but do nothing when the abuse continues, you are in an even worse situation because they know they have the upper hand.
So the choice is yours, 0opsidia. You continue faffing around, like the fly on the pin suffering and letting your persecutor get the upperhand or you take decisive action by preparing a dossier and going to the police.
My betting is that you will do nothing, just keep wriggling on that pin. My experience is that the most difficult abuse to deal with is elder abuse, because the person abusing the older person is a relative and they do not want to upset other members in the family. So they suffer, and suffer and suffer, like you are doing now.
I am sorry if this reads as a brutal post - no, actually I am not. I am being cruel to be kind. I am horrified by your situation. I would do anything to help you, but help can only come if you are prepared to stiffen your spine, say 'enough is enough' and deal decisively with the problem.
Ask yourself whether you really are prepared to suffer like this until you die, rather than upset family members? I cannot believe that is so.
Here is a link to an Age UK Factsheet www.ageuk.org.uk/globalassets/age-uk/documents/factsheets/fs78_safeguarding_older_people_from_abuse_fcs.pdf
I suggest you talk to someone independent. Start by talking to Age UK, ask them whether they can find someone who can work with you as your advocate (not a lawyer of any kind) but a person who will walk with you and be with you as you do all that is necessary and help you deal with the legal side of things.
At the end of the day 00opsidia the ball is in your court and only you can decide where to hit it.
Here is a link to an Age UK Factsheet www.ageuk.org.uk/globalassets/age-uk/documents/factsheets/fs78_safeguarding_older_people_from_abuse_fcs.pdf