She got three years for that. She'll have to declare that conviction for life, it'll never be spent. Seems the law considers repeated assaults to be less serious.
There have been changes to the Rehabilitation of Offenders Act (ROA), coming into force in March 2014. The changes were retrospective, so anyone convicted prior to March 2014 will also be subject to the new rules regarding periods of rehabilitation.
Someone given a sentence of more than 6 months but less than 4 years now has a rehabilitation period of the sentence plus 7 years, so in your sister's case, with a 3 year sentence, that's 10 years from date of conviction. If she was convicted more than 10 years ago her conviction is now spent and no longer has to be declared. If less than 10 years ago then she still has to declare, but, with it being a sentence of less than 4 years, it will become spent at some point. Previous to March 2014, as you are no doubt aware with your sister's case, a sentence over 2½ years (30 months) was never spent.
There are certain professions where applying for a job in them is exempt from the ROA. These are professions which are permitted to carry out Standard or Enhanced DBS (Disclosure and Barring Service, formerly Criminal Records Bureau) checks.
Also important to remember that it's not just about applying for jobs. Insurance cover is hard to find and nearly always adversely affected by unspent convictions. 10 years from the date of your sister's conviction she no longer has to tell insurance companies about that conviction. If an existing insurer knows about the conviction and a premium is being paid because of that, then it's worth trying to renegotiate cover once the conviction becomes spent. Or go to a different insurer who won't need to know about the spent conviction. I, being subject to the ROA until 2013, did just this. I got my home contents premium with my existing insurer reduced by 30%. This year I switched insurers and reduced the premium by around another 10%.
In all cases no matter how the question is asked, if a conviction is spent it is spent. If an employer or insurance company just asks, "Do you have any convictions?" or, "Do you have any
spent convictions?" you can say, "No." This is referred to as a 'legal lie'.