Mental disorder?

I am very worried about a family member who's mental health seems to be deteriorating but whom won't seek help. In all honesty I don't know what is wrong, the symptoms seem to fit a lot of conditions so finding answers is proving very difficult but maybe someone out there may have experienced something similar or be able to make suggestions or what could be wrong and what we can do to get this person help.
She nearly 70 years old and her behavior has been changing a lot recently. She seems to be living in the past, but not in a positive way. She is dragging up negative things from her past and making issue out of them today, things that should be long gone and insignificant given the timescale that has passed. She has been married for over 40 years but has suddenly become obsessed with her husbands ex girlfriend from when he was a teenager! (the ex is dead and has been for a number of years and he's now a pensioner who's not seen this woman in decades.) She is accusing her husband of all sorts and digging through old photographs from his past looking for evidence of things that she knows never happened but has somehow convinced herself that they have.
She's grilling him and their mutual friends about this ex partner constantly, even though she has known about her their entire married life. It's bizarre. Why this has suddenly become an issue now over 40 years later no one can understand, but it has and she is causing massive arguments all of the time because she won't let it go and he is understandably getting really annoyed about being hauled over the coals about a girlfriend he had before he met his now wife of 40 years! She's in this weird bitter, jealous, deluded place that none of us can understand and we've tried to talk to her but she gets nasty and can't seem to see a problem with her behavior.
She's also trawling trough social media and looking up pretty much every woman he has ever known such as former colleagues and grilling him about them too! She seems to have it in for every woman in his life, she has fallen out with several female family members and friends, the neighbour and even the Avon woman! She bitches about every attractive woman who appears on the TV. It's really toxic and concerning. She's isolating herself by being like this.
There seems to be elements of delusion in that she seems convinced all of these women are a threat (even dead ones) and massive amounts of bitterness and jealousy about mostly historic things. Her marriage is now on the rocks as are most of her personal relationships thanks to her jealousy of other women.
Some of the things I have read say this is symptomatic of the early stages of dementia (delusions, paranoia, mood swings, living in the past) but she doesn't seem to have any other symptoms? Could this be the start of it?
Could this be depression? A personality disorder? Some kind of breakdown? Delusional disorder?
We've all talked to her and tried to get her to see reason but within days she's back at it again and the behaviors are becoming worse and worse. She's doing batshit crazy things like playing songs to her husband she knew his ex listened to, taking him past where she is buried, reaching out to people she hasn't spoken to in years just to fact find, reading old diaries and throwing 40 year old argument at her husband like they are happening now and going online to try to find things out about his past that never happened. We've suggested therapy but she won't go. What do we do? She's scaring us with the intensity and unhealthiness of this behavior but how do we intervene if she won't get help? It seems to be getting worse weekly and we are worried about her wellbeing, and her husbands because it is bordering on bullying and emotional abuse the way she is behaving towards him now.