I don’t know what you can do about it, but judging by what you said your mum was like during his cancer treatment it sounds like your mum is the one being dismissive and lying, rather than your dad.
It sounds like your mum is neglecting your dad.

I'd assume the truth is somewhere in the middle. She's probably neglecting him but she's had enough of him not taking care of himself (drinking, smoking) and her having to pick up the pieces. I don't know how you deal with this, just listen to them but don't get involved, don't try to figure out who is at fault, it will make no difference in the end.

It sounds like your dad has been making your mum miserable for years and now its payback time.

Slightly different circumstances - my dad was abusive, mum not but wouldn't ever leave him so I had the daily hours long upset phone calls from her - but I think the advice will work just as well. I perfected "umm" "ahhh" "uh, huh" and other useful phrases whilst doing other things. I believe this is what handsfree is designed for. It may feel a bit cruel at first, but they are adults and quite capable of sorting their own sh*t. This has been going on for years, as you say - it either of them genuinely had enough of the drama they can still end it. They are simply dragging you into it. Don't let them.

What if you don’t believe either, accept that you can’t know and move forward addressing issues in the present. Perhaps ignore the aspects of the story that are about the he said/she said and concentrate on how your dad feels now, whether he needs help or medical attention right now.
Perhaps get him one of those alert things so that he can get help without her involvement if necessary.

It is exhausting dealing with parents who act like toddlers.

Could he be developing dementia.

You need to tell them to stop, unless it is an emergency and someone is close to death otherwise you don’t want to be hearing them squabbling.

It becomes a way of life for them and they won't consider your feelings at all.

My parents were similar, secretly miserable in my ear. I didn't mind DM venting as Ddad wasn't an easy man to live with but it was hard work.

He missed her terribly when she died, he died within a year, it was their way, they should have divorced years ago and became co-dependant on the misery.

Your parents won't change.

It sounds like after years of abuse your DM dosnt give a shit about him, I don’t blame her. It also sounds like he’s still trying to abuse her by going running to you and your sister with tales of her abusing him. Not to mention controlling you and your sister with his health issues. Personally I’d tell him to grow the fuck up and if he’s not prepared to seek the right kind of medical attention or adjust his lifestyle you don’t want to know. Same with their marriage, if they choose to live with each other that’s their problem. They are adults, let them get on with it.

So he got too drunk, passed out/got a bit wobbly and fell over, and your mum, sick of his years of shit, stepped passed him (as I would to someone who was drunk and had made my life a misery).

Now he's crying to you. If he's been abusive to her and is now feeling his control slip away through his own ailing health, he'll be very surprised that people aren't running to help him.

I'm possibly projecting my own dick of a dad, but I cannot wait for this day to come for him.

Some tough love required probably

'Dm /dd you're obviously not happy together, why don't you discuss a divorce. No? Then I'm sorry I can't listen to the moaning about each other'

'Your dm is abusing me' 'that sounds upsetting, if you drank less I think it'd be easier to cope with/make a gp appt'

Basically cut each story short and insert a repeated answer of how they can help themselves. Then swap to a chipper conversation. Hopefully they'll get bored of telling you if you don't give the right answers.

As the daughter of parents who are mutually abusive, I feel your pain. In my case dad only talks to me when he needs It sorting out or if mum is not there. Mum vents All. The. Time.

I do as someone earlier suggested and put on hands free and yes, uhuh, my way through a rant. I also have my mother threatening suicide. It's a cry for attention, has been going on for the last 35 years and I don't take it seriously anymore.

I have had the conversation with them both about divorce, and my go to line now is: it is your life, I can't live it for you and you need to make your own choices. If you are prepared to put up with each other then go ahead, but accept your choice. They hate each other, it is toxic, they are unhappy in their misery, together.

It drives me utterly crazy, have had a near nervous breakdown because of it and have now learned to distance myself and change the subject rapidly.

It looks to me like you don't want to believe your mum because you know how awful your dad is.
I feel sorry for her. She comes from a generation where you could not leave as you had nowhere to go and society would have looked down on you, so you shut up and put up with the abuse.
She needs your support. She's had a horrible life

It sounds like your mum is the abuser here - the pattern being your dad saying he's suffering but please don't tell your mum, then her denying the whole thing. Classic abuser behaviour, usually it's a battered wife who doesn't want to press charges against her abusive husband, but female on male domestic abuse is much more common than people think, especially when the male is physically weaker eg due to age or illness.

I think you need to take every allegation your dad makes seriously and believe him rather than your mum.

You are not their therapist, mediator or marriage counsellor.

You could just tell both parents " I'm sick of being pig in the middle. I'm not going to listen to either of you lying for sympathy, moaning about the other and expecting me to take sides. Every time you do this I shall just put the phone down."

Very old friends of ours (long bad marriage, both at fault) had a daughter who became a professional psychologist, and that was the ultimatum she delivered to them. They continued to hurt each other , but Daughter was no longer their stretcher bearer or emotional paramedic.

Some good advice here. It sounds very wearing OP, time to find a way to take a least one step back.

I think you need to assume that your mum isn't going to provide care for your dad and act accordingly.

What action would you take if she wasn't there? Could he stay at home with carer visits or would you be looking at residential care?

It’s is good advice.

Why are you even this involved?

Toxic parents that ruin each other's lives as well as those around them do not deserve a platform to take their grievances to op.

You should shut down the lies/complaints with a simple 'This is none of my/our business Mam/Dad please speak to Dad/Mam about this. It makes me uncomfortable' and move on.

Why are you enabling any of this? Maybe your mam has grievances and resentment from previous treatment, maybe she is tired of watching your dad slowly kill himself?

Step away. Focus on your own life. Leave them to it.

Keep your communication going with your sister.
Best skip the following if you sadly miss or are about to lose a parent.

I had a great chat with a friend yesterday, it was such a relief for both of us to say, we hope our dad's die soon. Both are shadows of their former selves and our mums are broken by the non stop caring for what have been long but not particularly well suited marriages.

TBH I feel bad for your Mum. He’s complaining but doing nothing about it. Falling (likely?) through drink and expecting her to pick up the pieces. This doesn’t strike me as neglect but accepting that he is not doing anything to improve his situation and so why should she waste physical and emotional energy on it? I suspect the answer is because you and your sister are — but that is your choice. He needs to take some responsibility for himself.

Sounds like your dad is an alcoholic and doesn’t appreciate the second chance he’s been given. What a waste of NHS resources! No wonder your mum is sick of him. I also wouldn’t trust the accounts of a drunk - no doubt they’re littered with lies.

This is how I interpreted it - and I have a lovely dad so I don't think you're necessarily projecting.

I think he's fallen over drunk and she's sick of his shit.

He won't phone the doctors because he knows they'll tell him he needs to stop drinking.

Your Mum didn't push your Dad to the floor, she declined to lift him up. She didn't make your Dad ill she declined to be his carer.
Your Dad made your Mum miserable for years, just because for whatever reason she was unable to leave that doesn't make her responsible for him or neglectful when she declines to be responsible for him. These are things you do out of love, she doesn't love him and that's okay.
Accusing your Mother of enjoying it/ neglecting him (as if her abuser is now her responsibility) must be incredibly hurtful to her. She is allowed to be indifferent to his needs after years of him actively making her unhappy that doesn't make her an aggressor.
If you want to look after him go nuts, no one should be telling a 70 year old woman who's done her time what she should be caring about though.
I agree with a pp he sounds manipulative and reporting back to you the natural consequences of making someone hate you is a massive pile of DARVO crap.

YABU to get yourself all bothered. They are as bad as each other. Leave them to it.