Twin 

That sounds really tough. It sounds like you can't make her work harder or differently but instead have to watch and be supportive if and when she needs a plan B.
There are lots of DC on this thread who are flying through but most DC don't. There are loads of good courses that don't have high tariffs but are still worth doing if you love the subject or it gets you where you want to go.
2 of mine struggled at sixth form.
One did 3 year sixth form. Still did underwhelmingly. Went to ex-poly. Enjoyed life in big city. Enjoyed most of course and 2 modules in particular which led to career choice not previously considered. Has new job which they love.
Another DC dropped out of y12 having got engineering apprenticeship. Worked in cafe until starting apprenticeship. Refused to do AS levels. Now doing HNC. No debt. Determined to do degree and PhD.
They are dyslexic and found it hard to perform in exams, organise things and got tired after school.