Oh dear - this is getting me worried. DS2 is expecting his second child. Its big sister will be just turning 4 when it is due, and is being very well prepared, and has always loved playing with her dolls and is looking forward to having a real baby to help with. As Doversole said, I would have thought that the 4-year-gap was ideal. I hope things don't take a turn for the worse when it arrives, although DiL is extremely competent (she is a nursery teacher for children with difficulties) and should be able to keep things under control.
The very same DS2 was 5 when his younger brother was born, and also enjoyed mothering the baby. I really can't remember anything about the relationship between my first 2 who were less than 2 years apart, so it can't have been very bad.
But, in the case of Grandelinquent's little ones: There's no way of knowing how kids are going to react, is there? But I would have thought that by 4 years old at least you can explain more to them and involve them in the baby's daily routine, so maybe it will soon resolve itself.
Also - at 5 years she is having to cope with a lot herself. I think of it as a pre-pre-puberty. They are just getting their 2nd teeth and hormones are messing them up inside. Like all other stages of development, some people are more affected than others.