How can I stop people from 'helping' ?

Just say “OK that’s a good idea” then do as you planned, some things are just not worth arguing about.

I have a friend who reglularly does b).
Not just to me but to others as well.
We all just smile.
She doesnt seem to notice we dont actually do all things advised. Everyone happy[as long as we can keep ignoring her].

Sometimes I don't understand her. Why didn't she think 'ok, she's got it worked out' and leave it, instead of losing her temper.
I did not lose mine, but now I'm really upset.

...and I will do it - but later, when I've cooled down.

My MIL would say “If you say so.” in a tone that brought the subject to and end.

“Thats a good idea” is gentler.

But basically a shit down and immediately follow with something completely different - preferably a question that they have to answer in some detail. “How is the cat?” or whatever.

Harder to deal with I found were the people who did things for without asking. It was one of the reasons I moved after my husband died because for some reason people intruded in a way they would never have done before.

Bought me equipment, did stuff in the garden, organised people to visit and told some others not to.

I totally understand your frustration.

Is she your older sister or has a bossy personality? Sometimes just can't see the wood for the trees. A woman at our WI is like that. She thinks she is 'helping'. I suppose you could say: Why would I want to do that? or I have been looking after myself for 80, why would I stop now?

Are you relying on her for a lift Alie2Oxon?

Otherwise this doesn't really make sense.

She tells you to go to Asda instead of the nearest cash-point, you say, "no dear, I've planned my day, starting at the cash-point... talk to you later, bye-ee"

As for the wider problem of friends offering 'helpful' advice - I understand, I'm 84 and bereaved last year. Well-meaning - and they usually are - friends and acquaintances frequently give me the "why don't you join xyz, you might enjoy the activities" or "I think you should zyx..."

It's frustrating, I know.

My close friends and family are aware that if I genuinely need help or advice, I will ask them and equally genuinely listen to their advice or take up the offer of help. Because they know that, like them, I have a mind of my own.

It ruins your day, I know, when you have it planned and something or someone comes along and derails it. Talk to your sister, quietly without anger, and tell her that your autonomy over your choices is important to you.

She's my younger sister by nine years. I suspect some ancient jealousy, but there's no way of talking about that...
Yeah, bossy.
I suppose I used to do what she wanted.

She may lash out at me verbally now (emails), but I do not respond in kind. I don't do hurting.
I think I need to be an immovable object.

I know it doesn't make sense!

I thought about asking her for a lift yesterday but decided I could make it easier by taxi today.
If I ask for help. fine. But I didn't...

I think there's some deeper discrimination here though... if your body is disabled, your mental capabilities must be as well?
I have a wheelchair and any time I'm being pushed some- where I have the impulse to assert myself - which I do, especially when someone speaks to the pusher first!

I confess to trying to be "helpful" on many occasions. Often it's just for the sake of making conversation. In return I welcome bright ideas and recommendations from other people.

Has your sister tended to be a bit bossy in the past? If so, never mind. At least she's still talking to you and you know what to expect.

When you're feeling patronised and annoyed by those who think they know best just change the subject.

Good luck with your book!