I have this too. It is pretty bad that she hasn't had advice on diet yet.
For me, my blood sugar is always higher in the morning and porridge, which should be fine, always sends my blood sugar rocketing. Avoid dried fruit in the porridge, as this makes it worse. I usually have bacon and/or eggs for breakfast with a small slice of wholemeal bread.
For lunch, I have a Covent Garden soup followed by a yoghurt. In the evening things are better and I eat a couple of servings of veg with meat or fish and a couple of new potatoes. Jacket potato with beans and cheese has also been fine.
I get very hungry between meals, so snack on things like cheese with apple, hot cross buns. I check packaging and avoid anything with carbs that sugar per 100g over 10g.
Avoid things labelled diabetic, as excessive consumption can have a laxative effect!
I'm 26 weeks and have maintained a controlled level of blood sugar, and average around 5.1. I am on insulin and metformin, and also breastfeeding which lowers blood sugar naturally. Your sister should also aim to exercise for 30 mins or so a day too.
I am having regular scans and my baby is on the 10th centile, so not big! I will be having a c-section, but a friend of mine is pg with her first and has not been told she'll have one. I haven't challenged it because I'm quite happy with the prospect for various reasons.
Hope this helps.