My heart goes out to our sister and to you, it is the most frightening thing ever. My DD had a febrile convulsion when she was 2, I thought she was dead, and it has never left me.
The important thing to tell your sister is that it is the speed of the temperature rise which causes the convulsion, rather than the height of the temperature itself. So, at the first sign of a fever, strip the baby down to her nappy, put her under a fan and give her the age-appropriate dose of calpol.
What I used to do was to alternate doses of calpol and nurofen so DD always had something in her system. Also, I would advise her to write down the time she gave each dose as when you have been awake for nights on end you can get very muddled.
It is true that your niece could continue to have febrile convulsions until she is 5, but it is also possible that she could never have another one again. As she is so vigilant with the temperature it is less likely. Once your niece is 5yo the risk of convulsions decreases dramatically.
Did the hospital identify what had caused the temperature in your niece?
Your sister is going to be very fragile for a long time, and really just needs understanding and support. I felt I was going mad with the worry. As far as going back to work is concerned, I would advise her to write down instructions as to what to do should DD get a temperature and give this to the person minding her.
I used to check my DD's temperature constantly but as she got bigger, the worry did subside a little. However, it never really goes - my DD is 17 now and if she is unwell now and gets a temperature my stomach gets in a knot and I get dreadfully worried, so I do understand.
I don't think other mums understood my anxiety over it at the time - unless you have seen your baby fitting it is hard to describe how terrified and helpless you feel. Feel free to pm me if you have any other questions or just want to talk about it - I know how difficult it is.