How do you make amends.

[PermanentTemporary]

I've hurt my sister. Multiple times. Something big has happened and it's all coming out. They're all true, and all to do with our relationship with our mother, though of course my perspective isn't quite the same. I don't think I can do anything just yet. How does anyone make amends for repeatedly being selfish and clueless.

I'm not sure that you can. It's difficult to say based on how vague your OP is, but the bits I picked up on are I've hurt my sister. Multiple times.
and How does anyone make amends for repeatedly being selfish and clueless.

The slight ray of light is 'clueless' which suggests you didn't realise you were repeatedly hurting your sister, but I have to say I am NC with my own sister because she's a selfish cow who repeatedly hurt me by her behaviour.

Can you give some more info if you want constructive help?

Yes, I'm afraid this will be terminal for our relationship.

Central problem is where my increasingly frail mum should live. Mum passionately wants one place that my sister feels isn't possible due to distance and frailty. Associated hurts I have done: progressed mum moving to that place despite knowing my sister disapproved, and my sister feels I hid this/my perspective is that mum wants it so much i will go along with it and try to make sure at least she doesn't buy a really unsafe property and also a hope that if mum sees what she can really afford she might decide not to go; been in touch with my mum's boyfriend behind her back - tbh it was only a few text exchanges about trying to help them meet, he got in touch with me as my mum has lost her mobile; being selfish when my sister paid for a holiday and my mum bought me a present and not my sister (she did buy a general present but paid for something personal for me and I didn't notice).

I think of my sister being slightly frightening and very opinionated but also very loving. I get passive around her but definitely can segue into passive aggressive or secretive. I fear her pain and outbursts but end up causing both.

Hmm. From this, I have to say I'm on your side, actually! Your sister sounds tricky. I don't actually think you've done much wrong.

You've supported your mother's wishes against your sister's disapproval. Who made your sister God? Why do her feelings about where your Mum should live mean that no one dare challenge this? You are perfectly at liberty to text other people without her permission - why are you not allowed to have contact with your Mum's BF?

I'm also not sure why it's your fault that your DM bought you a present on holiday or why your sister thinks this is selfish of you.

If it's possible I'd try to remain calm and say to her, I'm sorry you feel hurt, but we have different opinions on what is best for Mum and my opinions are valid too.

I'll be honest - you might be much happier without her in your life if she decides she's having no more to do with you. She sounds quite emotionally abusive.

I can see I've already put my spin on it. My sister feels blindsided because there's no doubt I didn't update her with everything I was thinking and doing about my mum - it's just that I wasn't being an organised betrayer, I felt caught between my mum and my sister and tbh genuinely agreed with whoever I was talking to at the time...

But that's perfectly normal. I think you are allowing your sister to put all the blame on you for 'betraying' her, whereas in fact you just were doing your best to pour oil on troubled waters and keep everyone happy.

I still think that she needs to examine her own behaviour. You describe her as slightly frightening, very opinionated and prone to outbursts.

She's clearly got your role defined as the Bad Betrayer, but you are doing your best for your Mum, in the way you think is most helpful. It sounds like Sister thinks everyone - including your DM - should do as she decrees. Maybe have a think about why this is?

Why is she automatically right? Your DM passionately wants to live in a place that DSis has deemed unsuitable. Is DM paying? Does she have dementia, and DSis have power of attorney? DM appears to have a bf and the money to purchase a property. You say she is frail - but as long as she has her faculties then ultimately it's her life and her choice. Your sister sounds bullying and manipulative.

However - even if she genuinely believes this place isn't right for your DM, and even if she is arguing this out of good intentions, it does NOT make her automatically RIGHT, and other people may disagree with her.