I think you can test for MODY. Genetics testing isn't cheap, and it may not be covered by NHS (you are in the UK, right?). But your daughter may very well meet the guidelines for when to suspect and test for MODY:
The 2014 ISPAD guidelines also indicate a diagnosis of MODY should be suspected in the following clinical scenarios:
--A family history of diabetes in one parent and first-degree relatives of that affected parent, in patients who lack the characteristics of type 1 or type 2 diabetes
--Mild stable fasting hyperglycemia that does not progress
The other times genetic testing is suggested is when a child is diagnosed diabetic before 6 months or between 6 and 12 months with no islet cell antibodies. However, MODY can also present later in life.
It might very well be worth talking to your endo about testing for the conditions. There are, I believe, 11 identified types of MODY now (maybe more), and they all have different treatment requirements (from Metformin to intermittent insulin use).
Another possibility (if it appears she really is diabetic, but not Type 2 and without antibodies) is "Idiopathic Diabetes Mellitus," sometimes called "Type 1b." It's a genetic disorder which sometimes presents as "mild" Type 1, and often requires intermittent treatment with insulin throughout life. There isn't a lot of information available on the internet, since it is exceedingly rare and mostly found in Asian and African families. From John's Hopkins:
Type 1b or idiopathic diabetes: an unusual form of phenotypic type 1 diabetes with almost complete insulin deficiency, a strong hereditary component, and no evidence of autoimmunity. Reported mainly in Africa and Asia.