Right hello Petes-dragon, this is REALLY important.
My son had slapped cheek syndrome when my wife was pregnant with our eight year old daughter and it can result in feotal death if the mother is not monitored. What happens is the foetus developes hydrops and could potentially die, so you MUST get your sister checked ASAP. Don't be fobbed off as very few GP's even know this. We were lucky and mentioned it casually to a midwife who had previously worked in the Foetal Medicine Unit at St Michael's Hospital in Bristol and she really moved on it. My wife had to go for weekly scans (U/S /dopplers etc) and was checked rigourously.If it's checked it can be treated and all will be fine.
If you can't get any sense out of her GP/ midwife about it PLEASE call St Michaels Feotal medicine unit in Bristol who will then contact your GP for you.
Also warn the nursery and any other expectant mother's as it's only truly dangerous to the unborn.It has to be taken really seriously in case it is fifths disease/slap cheek.
I hope I've not alarmed you too much, but please get her checked at a FMU ( her Dr should take a blood test to see if she has anti-bodies , if she does, she MUST be scanned).