to feel like I cannot become a substitute daughter for my elderly uncle?

You don't know they are shit. I know my mum says this about me to people when in fact I couldn't do more.

If you work, could you tell him that your employers have said you are no longer allowed to keep your phone on during working hours, then mute his messages except for an hour each evening? Or tell him your phone is playing up and you don't always get messages or calls promptly, then mute him until convenient.

This man is not so elderly he can't attend groups or socialise or start new efforts at life

My dad is 82 and has a more active life than I do

Your uncle has clearly not good a good relationship with either his sons or sister and I have to assume there are reasons for that.

You need to set boundaries than be absolutely ruthless. Agree to two chsts a week and then tell yourself over and over again that any more is enabling him to avoid being independent.

Btw I think your mum is letting you down here not stepping in to at least help you navigate the relationship with him

I agree with this. I think it’s quite apparent the other relatives have figured him out and put their boundaries in place. OP needs to learn to do the same. That doesn’t have to be blocking him and ignoring him completely but there is a middle ground.

I’m feeling a bit cross about some of the criticism of the other relatives, particularly OP’s mother. Why are her needs, wants, time, happiness less important than the uncle’s? He’s not incapacitated. He’s elderly, presumably so is OPs mother. He’s widowed and has cancer. He’s lonely but doesn’t do anything constructive to address this. These are not an exceptional set of circumstances for someone in their 70s. They don’t mean the person’s wishes override everyone else’s. And the brother and sister don’t get on. In what world does the mother need to put her wellbeing second to her brother’s? I just don’t get it.

Yes, no wonder he has always wanted a daughter....to care for him! You are very kind, OP.
At least he does live a long way away or you can bet your life you'd be expected to care for him physically too! I guess at least you can be in control of how much or how fast you answer his messages, whether he likes it or not.

But you're a nice person, so you're going to feel guilty whatever you do! But you do need to put firm boundaries in place...perhaps then your cousins will step up. Good luck!

Set boundaries or it’ll get worse. He’s lonely and elderly but still needs to sort out his own social life, he can’t expect you to do it for him.
Tell him that you’ll only be responding in the evening if you have time, reduce this as you go,
He doesn’t care about your feelings only his own.

Also where have you got the idea he’s dying from?

OP has not said he is, she’s said he’s in his 70s and the cancer is slow growing.

It's her brother - she's had an entire lifetime of his neediness and can't be arsed with it.

Just because she's retired doesn't mean she has to be on tap for his nonsense.

His own sons aren't stepping up. That doesn't mean the women of his family have to. He's in his 70s, not his 90s.

I feel so angry for you OP. I would send a message to the sons and him on a group WhatsApp chat and explain again that they need to step up. And say that any future requests have to go on that chat and that chat alone, or there will be no response. Ostensibly so that whoever can reply first, will.

Or once a week

Well it won’t work will it? You know nothing of the history between the uncle and his sons and sister, or their reasons for keeping their distance (although his complete failure to care about other people’s needs and lives could well be one 🙄). So acting as his flying monkey (as described above) is just going to be more trouble and stress for OP.

She needs to concentrate on deciding how much she wants to do and learning how to stick to it.

This is beyond inappropriate. It’s also committing Op to be the one to continue supporting and caring to the extent she won’t even tell anyone if he dies!

Why is it cruel of the sons and OP’s mum? They’ve set boundaries. They’ve obviously had enough. The OP hasn’t set boundaries.

I’ve had a relative like this uncle and I know how draining and all-consuming it is. People back off for their own sanity - not to be cruel.

Edited to add, he’s possibly not dying either. He could go on for another fifteen years, if not more.

I know it’s not nice to shame them but they really need to understand the situation tough love is called for. They are going along single no real responsibility apart from a job but they ignore there dying dad it seems like they need a kick up their arses.

Op he shouldn’t need a befriending service I understand why he doesn’t want it he wants his family. I would send all three of them a message saying I have too much on my plate to help your dad/brother so if you want him to die a lonely man that is up to but if something happens to him this is on you and if he does pass away and you don’t care for him as you should I won’t be informing you of his death because if you don’t care enough now when is alive you won’t care about his death. You should all be ashamed of yourselves call yourself family. Disgusting. Op it’s not nice but seriously they are horrible human beings.

Op honestly he knows you will cave that is why it’s you. It’s not being horrible it’s your life but he has 3 other people who are technically immediate family yet there happy to ignore him that says a lot about them. I could not imagine ignoring my dying brother yes he’s lonely but that’s not on you. If it were my uncle it would be now and again check ins but siblings would be a lot more. Do his sons and your mum not like him? Because it seems very cruel behaviour of the 3 of them.

‘No family is nearby’ then if I replied to him, I would suggest he needs to pay someone local to help.

This is such an important lesson @Blaainey it doesn’t have to be all or nothing and to be honest I suspect you are just behind his sons and your own mother in realising this.

It is my experience that people who cannot tolerate setting boundaries themselves have huge issues with other people’s boundaries which they struggle to tolerate too.

Realistically though the choice is boundaries or burnout.

OP you keep focusing on what everyone else is or isn’t doing - that’s not the issue here.

His sons being single or having more time doesn’t matter, they have made their choice about how much they want to engage with him.

Your uncle knows there are other service available to him, yet he chooses not to engage with them.

You have to make your choice - you have a choice to step away and put in place boundaries, like everyone else has done.

There’s no point in resenting them for doing what you’re unable to do.

Someone on here said some very wise words; if you have to choose between guilt and resentment, choose guit.