Hi Aliasme
A very difficult situation.
Just to let you know that it is very common for children and teenagers to use 'denial' as a way of coping with the stress of this situion. It may seem she is being blase - but I suspect she isn't at all - but it is her way of coping with a very distressing and potentially shameful situation.
There is a condition called encopresis which affects bowel control - it is VERY common for children to go to exremes as to soil themselves and not do anything about it. This is a stress and shock response.
It might be more helpful for her to talk to an incontinence nurse - they would be more matter of fact - and it is often difficult to talk to family as too shameful and involved.
You sound very supportive and caring to your daughter.
Another thing is Tena type pads are better than sanitary towels as they are much more absorbant of urine - perhaps they could be taken out of the packet or she could tell her friends they are sanitary protection (at her age lots of kids wear towels). However it may be she is resistant as that is part of accepting the problem.
One of the issues your daughter might have is a fear of what is happening to her.
I think it might be good to plan, and even rehearse with her, the following:
What to say to kids / teachers if someone notices she has wet herself (she may not know what words to use or the appropriate explanation to say).
What to do if she has an accident out and about- eg spare clothes, knickers etc - where they are (in her bag) what to do.
She also needs a clear biological explanation of how her bladder works - and why she needs to drink etc. This may not all go in at first - and although not necessary for her to know all the details from a health perspective - it can help psychologically - by externalising what is happening. Even kids her age can tend to blame themselves somewhat - and it is very important she realises this is a medical issue - not a personal failing. If she understands the mechanics of how it all works physically - even subconciously this helps externalise the problem from herself.
I'm not a professional, so please feel free to ignore any of this I may be saying something which is not appropriate to your daughter. I have had some experience of this though -and these things have helped other kids.
Good luck to you in this difficult and confidence sapping situation.