Hi Susie,
Sorry to hear about your sister in law's ill health. In relation to whether she can claim social welfare it would make a difference if she were pre 95 civil servant or post 95 as she would be paying different types of contributions. Broadly speaking if she were pre 95 she would be paying B cons and therefore probably only entitled to claim something like Disability Allowance which is a means tested payment. If she is post 95 she will be paying A cons and therefore could apply for Illness Benefit and perhaps further down the line Invalidity.
Best of luck to all,