Hello there wooker, unfortunately, your options are pretty much limited. I one think that you have only 2 options: the first one is that you can either go with a sensor that it is requiring blood or the second one is that you could go with a continuous glucose measurement system.
Well, a continuous glucose measurement system uses a sensor that you insert under the skin and leave in place. Anyway, I have to say that I am not an very big expert on diabetes, however I do believe that those are generally measuring interstitial glucose (which, to my knowledge, is changing more slowly over some the time) instead of the blood glucose, that is why your measurements are going to lag a little bit behind the actual blood glucose levels.
Also, I think that the only completely non invasive method for you (the only one that at least I know about) is the Gluco Watch, which has been withdrawn a couple of years ago.
And yeah, I am not sure how helpful this would be but you could get talking Glucose Monitors for the blind, in case that would help. not sure if any of this would help but I really hope so. Good luck to you and your sister.