Sorry to hear about your niece. It is worth asking the hopsital about iv retinol- reduced complictions and severity- there is now published work on this.
I have written on here before that this (infants catching measles) is my biggest fear with the introduction of vaccination. In the pre-vaccination days adults were immune to measles (couldn't escape it basically- lifelong immunity etc) and so passive immunity was passed onto babies. This generally lasted 12 monthish and so it was rare for babies to get measles. Now the groups who are getting measles are infants (whose mothers didn't have measles) and adults- the 2 groups who used to be immune and who have the highest % of complications.
Unfortunately MMR is not all that great at producing immunity (and with the current schedule wears off anyway- still say they'll have to introduce regular boosters). My own son caight rubella from a child who had had MMR (which was why his mum didn't realise he had it- and spread it everywhere). Measles outbreaks have been recorded in populations with supposedly 100% immunity.
Veggie- most complications are recorded if children are deficient in vitamin A. This is why measles is such a killer in developing countries. Published research has now shown that giving iv retinol to children with measles reduced the lenght of time it lasts and the severity of the disease. However it has to be retinol that is given (not carotenes) I just mention that because of your name-(retinol is from animla products). In our case ds1 is given a daily dose of cod liver oil anyway for the contains vitamin A (lots of autistics have been shown to be low in vitamin A which is interesting given that the MMR damaged kids have measles virus in places they shouldn't be- hasn't been looked at much though). Last year I thought ds2 may be getting measles so I gave him a few doses of cod liver oil as well- just in case.
There is a list somewhere (on the internet?) where you can register to be told if another child on the list develops one of the childhood diseases. if one does then you can go for a tea party. If your child has had a measles vaccination- the best way to top up immunity is expose her to someone with measles. if I ever find the list I'll post the details! (maybe the informed parent, or jabs would know?)