Newbiegran, sounds all very good. Back in the past it was the absolute norm for all first time Mums to stay in hospital with babies for at least ten days.
The doctors should be keeping you daughter and SiL well informed as to any concerns they may have. If they having nothing to say, probably means nothing really to worry about, and they are just taking good precautions as they have noted some sort of infection.
Soon, they will be home, and all the usual worries will be there. Babies are a worry - but they grow up into healthy and active children. So pleased she has establised breast feeding - the best thing for babe (and herself). If he is feeding well, probably little to worry about.
Let us know how you all get on. Congratulations to all concerned.