To think this nurse was unprofessional?

The readings. Do the low readings agree with nurse’s one or the high reading agree with nurse’s one.

To simplify - OP’s family own two machines which are giving two very different readings (for just one family member so far but I think that’s a bit of a red herring - one of the machines is not working properly for the person who needs it)

To check which was the most accurate surgery offered to check against a manual reading.

Nurse checked one machine against manual reading got identical results so has identified which of the two machines is reading correctly.

OP is upset that nurse announced before starting that she wasn’t going to bother testing both.

I suspect that the nurse has experience of many of these machines and accurately predicted which the more reliable was going to be.

All that matters now is OP knows she has one machine which is working as it should.

I am not in the UK, but surely if you are worried or have potentially high BP that requires monitoring the GP would put you on a 24 hr BP monitor to.have a true assessment of your or your Sisters BP.
Would it be worth asking g for this and then starting fresh with a new home monitor

Exactly

But she checked one, it was correct, so the other one must be incorrect.

I spend a lot of time measuring blood pressures and interpreting those results in different contexts. It doesn't matter if today's readings were high - you would probably expect it to be higher because you were at the GP practice. What matters is that your machine correlated with the practice one. Now, when you go home you know that that machine is giving useful results. And the numbers may well be slightly lower because you're at home and that's fine, but, if you took the nurses machine home it would also give lower numbers than today because you'd be using it at home. If your readings at home are lower it would suggest that today's numbers had a white coat element to them. But, if your readings at home using the machine she's validated are as high as they were in clinic then you have hypertension that needs addressing. When it comes to interpreting the numbers the GP probably will look at what it was at home and what it was in clinic, but the thresholds for making a diagnosis are different for readings taken at home vs at clinic - they're not idiots. There is, fwiw, a school of thought that a white coat response probably isn't as innocent as everyone on MN thinks, but the GP will know that and will be able to use all of the results in the context of the rest of your medical history to come up with a plan for you. What matters most right now is that the nurse has said she believes the machine of yours that she tested is reliable enough.

Did you ask if that was on a nurse or doctors advice

The nurse checked your sister’s blood pressure on the surgery monitor then checked her blood pressure on one of your monitors. As the readings were similar then she didn’t need to check your other monitor as the nurse knows her monitor is accurate hence her telling you to bin the unchecked one.

Which the nurse did. You just do t believe her. I see you’ve been asked before. Higher or lower than yours?

This is clearly the issue. Can’t believe it’s being discussed

Why have you got 2 machines? What is your actual complaint, that the nurse falsified her reading? You preferred to have your BP checked 3 times unnecessarily just to see which machine was accurate because there's a difference in your sisters systolic, who wasnt even there at your appointment , when she takes it at least twice? Maybe an hca shouldn't be triaging patients, you may have got a different answer from a nurse or doctor,

Because the post and the updates are convulted, if the two machines give you two almost identical readings, and you have high blood pressure, it's totally irrelevant what they show to anyone else, and for another person to book appointment is, again, a total waste of time. If you are worried about your bloos pressure you book the appointment to check, but there are no issues with the machines for you, so again, it's a non-issue rather as you get correct readings.
Good you didnt send your sister to A&E with this.

I see. Well in the GP practices blood pressure monitors are calibrated every year or less. Do they maybe have a monitor you could borrow ( usually a small deposit is required). Or just buy a brand new one which will have been calibrated prior to leaving the factory. That way you can be sure.

So why are you and your sister going to the GP together? Are you not adults?

Ok, and can you answer my previous question, what do you mean by a manual check? Was this an independent check using their devices?

I don’t know about anyone else but my bp is rising.

So let's see if I've got this right. The nurse took a blood pressure with one machine. She then took a manual blood pressure (a very quick process) which matched that machine. So she then told you that machine was the accurate one, because it matched the manual BP. But you won't now use the confirmed reliable BP machine because you've got your knickers in a twist about the machine she didn't check, because she'd already proved the first machine was the accurate one.