'6th form' usually means studying for A-levels (or, at least, for AS levels). Some school 6th forms (and 6th Form Colleges) now offer much more but, on the assumption that only the traditional model is on offer to your niece, 6th form almost certainly isn't for her.
She needs to be looking for a place at a Further Education College and seeking a course suitable for her interests and likely qualifications.
Here are the entry requirements for the various courses on offer at 'Suffolk One', which is one of our local colleges combining both traditional 6th form learning with some of the BTEC courses traditionally offered at FE colleges:
http://www.suffolkone.ac.uk/courses/entry-requirements/
You'll see that entry to Level 3 BTEC diploma courses only requires 4 GCSE passes, with either English or Maths at Grade D being one of them, but with the others only requiring a Grade E. There aren't many such courses (at Suffolk One anyway, but there might be far more elsewhere) but something listed here might be of interest to her:
http://www.suffolkone.ac.uk/courses/diploma-courses-level-2/
A traditional FE college, such as West Suffolk College, offers far more courses at Level 1 or 2:
http://www.westsuffolkcollege.ac.uk/courses/course-search.aspx?mo=Full+Time
Part-time education might also be a possible option for your niece. A young friend of mine didn't achieve any great results at GCSE but he still got onto a plumbing course at West Suffolk college that was really designed for 'sandwich' students (i.e. with part of the week spent at college and part learning 'on the job'). He actually spent the first year of the course spending part of his time at college and part of it working for Tesco. However he had to find a part-time plumbing job (instead of working for Tesco), for the second year of the course, which I helped him to do. He's now 29 and a successful, fully-qualified plumber. Something similar might be of interest to your niece. (It doesn't have to be plumbing, although there's obviously no reason why it couldn't be. There are plenty of courses in what are traditionally regarded as girls' studies, such as hairdressing).