So sorry to hear about your niece and what a hard time she is having of it. My daughter is also 12 (yikes almost 13) and was dx'ed in February after being hospitalized with severe malnutrition, blood transfusions, TPN you name it. .
I honestly have never heard of anyone who was brought into remission within 4 weeks (unless by surgery) so while it is gut wrenching watching her suffer I would think it would be too soon to think remission. However, I would have hoped the meds they gave her would have brought some relief but this disease is so highly individualized it sometimes takes a while for them to find the right meds and doses etc.
Crap food...well that is half right according to our experience. Our daughter was told all white refined foods, white bread, white rice, fruit loops and frosted flakes cereal etc. Unfortuantely the healthy stuff requires the GI tract to work too hard so crap it is. That said, I would not have said all the crap you want...certainly not greasy fried garbage food!
My daughter was put on Prednisone and Remicade right away...after we weaned her off steroids it was apparent the Remicade wasn't working all the way. The doc suggested adding Methotrexate but I asked about exclusive enteral nutrition and that is what we went with. This is where the kids eat no food for at least 6 weeks and get all of their nutrition for a formula. So see you weren't that off on your bowel rest comment. They are allowed water and a few here have been allowed clear broth, noodles etc but my daughter was only allowed to chew a piece of gum a day. You can drink the formula in the form of shakes or have the formula delivered through a naso gastric tube. This trearment is the first line of defense in many countries but not usually here in the US. It is as successful at reducing inflammation as steroids and even better at mucosal healing. The extra added benefit is all the good nutrition the kids get and their added weight.
If you are interested, there are a few threads about EN. You may want to start with the Parents of Kids on EN thread here on the parents forum. There are also threads on the Treatment forum.
When you get a chance post where her disease is located and the meds tried etc and I am sure someone on here will be able to offer a lot more guidance.
Hang in there...the first few months are a blur and a lot to get used to but it does get better!