Hi
@RhiC and welcome to the forum. The reading of 6.8 on your MiL's meter would be the equivalent of 41 on the HbA1c (ifcc) scale. On that scale anything below 42 is non-diabetic, 42-47 is pre-diabetic, and 48 and above is diabetic.
However glucose meters aren't 100% accurate, and finger prick tests only give a snapshot of your blood sugar level at that moment in time, which can be affected by various factors including tiredness, stress, infection and what you have eaten recently.
The full HbA1c test, which is done with a blood sample sent to a lab for analysis gives a more accurate diagnosis as it measures your blood sugar levels over the last 8-12 weeks - the length of time it takes your red blood cells to be replaced.