I feel I should be posting on a board about toddlers not elderly parents.

[MissMarplesNiece]

I have never known such a performance from a grown woman. DM is diabetic. Her blood sugar is out of control. She helps herself to biscuits & chocolate bars etc. Every morning she claims her blood sugar is so low she needs 2 spoons of sugar in her tea, this is despite her blood glucose readings of 12, 14, 18 and even 21 this week (normal is between 6 & 7). Low is below 4.

Yesterday she had an opticians appointment. I got a frantic call from nephew (DM lives with sister) because DM was having a temper tantrum because she was refused sugar in her tea (glucose 12.5 before breakfast). She was refusing to get dressed and wailing like a banshee. She was vile to my BiL and caused my sister to be late for work - keeping a sitting judge waiting. My nephew recorded part of her tantrum because I thought I wouldn't believe him. It was truly awful, when I say wailing I'm not exaggerating.

When I challenged her she said they're all liars, she never has biscuits or chocolate. She's never asked for sugar in her tea etc.

I feel (and my sister) that I am at my wits end with her. If she was a toddler we could make her sit on the naughty step, or whatever parents do with toddlers. A grown, adult woman who has agency & capacity but choses to behave like a spoilt child? What can I do?

It's totally reasonable. The sick old woman isn't treating her family well is she? Expecting to be able to ruin her health then force them to deal with the consequences, ruining their lives in the process. That's really shitty. It's not unreasonable for the family to walk away from that. If sick old woman wants to behave reasonably I'm sure her family will be reasonable in return. Just because she's ill it doesn't give her a right to screw over others and have everyone run round after her when she's unwilling to help herself. They're her family not her skivvies.

Please speak to her diabetes nursing team about relaxing her targets. A healthy range for someone with diabetes in middle age is not appropriate for someone old and frail.
Blood sugars of 6 to 18 may well be fine for her. Over the age of 65, blood sugars will naturally be higher. - a normal reading for you could be a hypo for someone in their 80s. Also, the risks associated with hypos are much more of a concern for elderly people than the risks associated with high blood sugars.

I suppose I could say "ok then, no more opticians appointment etc" but I feel I do have some obligation to help her look after her health. In the same vein my sister could say help yourself to whatever you want, the consequences are on your head. Do we let her go blind, lose her feet etc?

Apart from trying to reason with her, yes that's all you can do. It's understandable that you'd want a way of preventing yourself from having upsetting feelings, but you can't do it by controlling her because that's abusive. Your feelings are your responsibility, the obligation you feel doesn't exist in reality. Her behaviour and the consequences of it are her responsibility. It's not all or nothing, you can help as much as you're able to without wrecking your own sanity, ruining your life or being poisoned by feelings of resentment. Just because she needs help doesn't mean you have to provide it, it's your decision what you do with your own time. Same as it's her decision what she does with her's.

"My mum is 88 & very frail. She has gone past the stage of being able to live in supported living. She needs nearly 24 hour care but neither my mum or my sister want her to go into a care home. She's been T2 for about 40 years."
If your mum doesn't want to go into a care home, she's going about it the wrong way. How would your sister manage if your mum lost her eyesight, or needed amputations? Hoists, remodelling the house, and damaging her own spine and health? A care home will be the only option then. So why isn't it an option now? All options should be on the table. If your mum wants to endanger her eyesight / limbs / lifespan, fine. But your sister does not have to facilitate that for her.