I don't know what kind of company you work at, but it sounds distinctly dodgy.
If your sister is in a discplinary process, then she has a right to have somebody to support her at any hearing. She also has the right to be told in writing of what the issue the employer has. She also has the right to put her side of the case and this may include her lack of training and educational issues, although it might be worth checking if the company was aware of these at the time they hired her.
She clearly should not go into any disciplinary meeting without a companion (see
http://www.direct.gov...rocedures/DG_10028115 ) for some details on this. Does she have a trade union or employee rep who could go along with her? You might be tempted to do this yourself, but you may be too emotionally involved, so it would be good to get somebody who can be calm and collected.
It is difficult to deal with issues such as in our second paragraph unless you have some proof of this. I would be inclined to concentrate on the main issue - what the reasons are for them to start disciplinary action against your sister.