Welcome Susan Cooper.
We're not able to give medical advice on these boards, not least because we don't know all details eg no idea of your niece's age. However, some families have found that their children with type 1 diabetes suffer constipation because either they are dehydrated, which can be treated by drinking more sugar free drinks) and / or they are not eating enough fibre, perhaps because of reluctance to eat enough fruit, cereal etc, which can cause blood sugars to peak, although as long as insulin doses and times are matched to carbohydrate intake, there's no need to limit these. If these measures of increasing fluids and increasing fibre don't work within a few days, it's time to go back to GP to ask for further guidance / investigations etc.
