(2 part post):
Firstly, your sister should ask her solicitor to arrange for a 'conference meeting', where both her solicitor and her barrister are present to advise her. (Far too many defendants never get to see their barrister until the day of their court appearance. There are some important legal considerations here, which need to be discussed as soon as possible).
The solicitor (and/or barrister) should be able to advise upon the possibility of getting the charge dropped from 'GBH with intent' to 'ABH'. Her partner's injuries would seem to a borderline case. See here for the definition of 'ABH':
http://www.cps.gov.uk/legal/l_to_o/offences_ag ainst_the_person/#P189_14382
Scroll down to read the definition of 'GBH'
Scroll down further to see what adds 'with intent' to a 'GBH' charge. (There's no such thing as 'ABH with intent').
'Section 18' is an exceptionally serious charge. It's akin to 'attempted murder', and carries the same maximum penalty of life imprisonment. (ABH, other than when racially aggravated, has a maximum penalty of 5 years imprisonment).