I didn't have diabetes yet when i was 15, i was diagnosed days before i turned 19... But if i had been diabetic when i was 15, i'd most likely be quite pissed off if my mum was naggin on me all the time about diabetes related things, especially if living with the disease for so long. Maybe it's time to let go and let her deal with it on her own. After all these years she should be able to manage it brilliantly as long as puberty doesn't cause too much trouble for her. Also, some, or maybe even quite many diabetics can pretty much feel at about what level they're at. I know of Type 1's who ONLY test when feeling completely off and don't know if it's a high or low causing it. The rest of the time they just go by feeling.
Get your hands on a meter that records the test results, the OTUS is a great choice in this case.
But the main thing with kids in the teens... Encourage, but NEVER nag about things. Nagging usually just means the teen will do the opposit, so if you nag on your daughter to test, she'll most likely test even less. I know i was that way anyways

If mum told me to clean my room, i watched TV instead

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22 years old, diagnosed T1D on october 14th 2004.
On MDI, Novorapid and Levemir, using the NP4
Currently back to pumping with my IR1200, April 2008.
Been using D-tron and Animas IR1200 but prefer the pen