To feel sad I'm going to end up 'managing' another man

Your Dad is looking to you to supervise this Man-Baby when he's gone. Exactly - he'll be gone. But you will have responsibility for the Feckless Uncle as well as your own life and matters therein.
You are being designated to spoon-feed this guy who, by the sound of it, will be pretty content to use you as a safety net, having 'form' for this kind of behaviour with others in the family.
As soon as your Father has passed then don't feel bad about ripping up his letter of wishes. It's unfair of him to try and direct your life from beyond the grave.

I see a problem here....How is the estate actually left? is it all left to you or what? It seems to me to be a very messy setup. I get the secondary executor in case the first one predeceases the testator thing, I have got a similar arrangement in my will. I also get the way this would niggle, that your father is binding his burden on you. The options that I see are
that you can tell your Dad "no"
that you can not say anything to your Dad and then decide what to do after he dies
that you can not say anything to your Dad and ignore the letter of wishes

How savvy is your "uncle"? and how much does your dad support him now? I know its a reach and you'd need to get advice or encourage your dad to do so but is there any possiblity that your uncle could challenge the will on the grounds that your father supports him and he is therefore a dependant?

Well you will need all the money , won’t you? So uncle will not get anything.

I wouldn’t give him a dime. This family conditioning to pander to this man has allowed him to be a baby all
his life. He probably had more than his share of his own mother’s money.

OP, I understand your feelings perfectly. It all depends on interpretation of the word "need", and if you are a conscientious person this will always be hanging over you.

If you have the sort of relationship with your DF that would make it possible, I would explain to him how you feel, that it’s not about the money but the uncertainty it leaves you in. What does "need" mean? What if you "need" it not to buy food but to move into a nicer house, or into a more expensive area, or would like an expensive holiday, or to pay more into your pension pot? it is not fair for your DF to put the decision and associated guilt onto you.

It would be far better for your father to either leave your uncle a lump sum (and if he wastes it that’s his choice) or, if he insists, to direct you to pay uncle a set small amount each month/year for the rest of his life. That way, at least you would know where you stood.

Your father had, presumably at least in earlier life, a strong relationship with his brother. You don’t and never have had, and it’s unfair if your father to be tying you into one like this.

Friendlycat has beat me to it. For me, the arrangement is far too woolly. If your dad left you a sum outright, you'd be free to choose what you do with it, whether that's buy a bigger house, invest it, take lots of expensive holidays or buy a luxury handbag collection. Whatever. It would be entirely your decision. With your Dad's arrangement, who's the arbiter of "need"? Will you have it on your conscience that you're off on holiday while your uncle is out doing a nightshift to pay his gas bill? If he gets wind of the LoW, will be be pestering you and asking you to justify your expenditure (whether it came from the inheritance or not) if he's struggling? If he has CF tendencies already and you're not on the best of terms now, it's a recipie for a lot of angst. Your angst. I'd try and explain this to your Dad and (if you feel able) come up with a solution you're both happy with.

Absolutely not.
Dont know how old you are but you're Uncle is 62. He is old enough to look after himself by now. If your father wants to look after him while he is alive, thats great but its not your responsiblity to look after him. The guy is 62 fgs. Time to grow up. He could live another 20 years. Like you say, you will end up another enabler as per all the other women in the family so no dont do it and dont feel bad about not doing it.
I imagine youre still young and its hard enough just trying to take responsibility for your own life and any immediate family you have. Let the cousins do it seeing as how theyre closer to him.. Besides a letter of wishes is just that, Wishes. Youre not legally bound by it.

I get what you mean OP, take the money you need, if any left then I would follow your Dad's wishes (then it won't play on your mind). To discharge the responsibility asap I would buy an annuity for your uncle that pays monthly if possible - then close that door. If your uncle hasn't got any SEN / Disability then he can take care of himself.

Absolutely this. Your dad is taking the piss with this. If he wants his brother taken care of, it's up to him to earmark money now.

No way would I do this.

I wouldn't be helping anyone who treated my mum like that.

But you don't want him written into his will either. If he leaves him £50k and you the rest of his estate and then care home fees eat into his estate leaving only £40k then he will get the lot. That clearly isn't what your dad wants.

I'd just leave it as is, and then do what you want when the time comes.

You set up a direct debit and then forget about it, after you’ve made it clear to uncle that there will never be more than the monthly amount.

It doesn’t have to be a mental load.

I'd tell your dad if he wants his useless, layabout, user of a brother to piss away all his hard earned savings, then he'll need to give it to him some other way. YOu're not comfortable or interested in getting involved, and it hurts that he's prioritise someone who happily wasted his life and mooched off everyone else instead of his own daughter who worked hard to stand on her own two feet.

By trying to be kind your Dad is setting up a terrible situation for you, and so I think you should be quite forceful in explaining to your Dad why this is a totally misguided proposal.

He is kicking the can down the road and loading you with the moral responsibility for your uncles welfare. The supposed freedom of “you don’t need to give him anything if you need the money” is setting you up for having to make a prioritisation decisions about your money for the rest of your uncles life, not just a one off. Any fun you have in your life your Dad is expecting you to navigate offsetting a guilt trip about your uncles quality of life.

Your dad is indulging his conscience with cake and eating it.
He needs to make a moral decision himself. Decide how much to leave to his brother, which could be in trust. Beyond that there is the welfare state.

If the will has only just been drafted then you are still taking this in. Hopefully in a month or two you will realise you have the freedom to do as you please with the money. It sounds like a sop to your Fathers conscience doing it this way which I understand and far better than seeing your Uncle getting a lump sum after a life of entitlement

It's all rather open ended and open to personal interpretation.

For instance a monthly stipend to cover care of your uncle could involve his monthly rent and all utilities plus living allowance - at what price would this be?

You needing money from the estate and putting yourself first can mean any number of things. Paying off your mortgage, enjoying some extensive travel and a bit of luxury.

Is this something you can talk to your father about to gain further insight into what he actually is suggesting within his Letter of wishes? Or is he just passing the responsibility and decision over to you?

Look if he really wants your uncle to be ok then he sets it in his will that there is a trust fund.

You are right OP, the expectation of some men that we have to take care of them is extremely irritating.

It’s a letter of wishes not the ark of the covenant, just ignore it. Surely you ‘need’ any money left by your dad for some sunny holidays?

She doesn't have to. As I said its a letter of wishes. She could just say there's no money for uncle at all.

But since the money isn't hers until her dad passes away, it's what he would have wanted. Its down to whether or not she wants to honour her dads wishes. Of course if dad puts a set figure in his will for uncle, then when uncle passes, the money will never revert back to OP. What if dad puts a figure of £50,000 for uncle? Whereas at the moment, the present will shows she maintains control, how much and if she wants to part with any money at all.

I wouldn't bother explaining anything to your dad. Let him make his will and write his letter of wishes. It will make him feel better.

Of course he won't be around any more by the time his will and letter come into play, at which point you do not have to step into the role of rescuer for his no-marks brother.

There is absolutely no benefit (to you) in tying yourself in knots over a piece of paper that will essentially become meaningless when your father passes.

Don't worry and don't beat yourself up.

The letter says that if you need money from the estate that you should put that need first. I would focus on that.

@twentyoneteacups a letter of wishes is simply that! a letter of wishes!!! it is not a letter of necessity. I, if i was you, would be "needing" the money to secure your own future and that of any children which you may have. I wonder if your father is actually giving him a monthly allowance at the moment???

You aren't overthinking things, you are just not clear about what you want.

Either your dad wants to set aside money, in which case he needs to do it or he doesn't.

If you don't say or do anything now, you will be in this forever. It isn't his place to sign you up for a lifetime of involvement. Also, why do your cousins need to be secondary executors? Are you not able to do this yourself? This all seems like a shit show of crossing over complications and confusion.

That's the problem with it for me OP, too many if's and but's and decisions for you to make when it should be up to your dad. He's inadvertently passing the buck and making you responsible when it should be his decision what happens to his money, and I don't blame you for not being comfortable with that.