My 15 year old neice has been having nightmares or night ...

There are several factors here that could contribute to the nightmares.
Decongestants used in some sinus treatments can be stimulatory and a common side effect is nightmares or disturbed sleep.
The recent poor results will be playing on her subconscious and causing her a great deal of stess if the results were not what she anticipated. She needs to spend some talking with soneone to establish whether these results are a one off occurrance because she didn't spend enough time on her studies or whether she needs a bit of coaching in that specific subject.
Sometimes A students do take on a lot and they give themselves additional pressure especially if they need a specific grade for university. She might need to be realistic about how much spare time she can spare for extra-curricular events.
Finally she might need to talk to someone about the loss of her Grandmother to establish if she has gone through the grieving process.
As you say, adolescents have a hard time coping with all the stresses around in their lives and sometimes they just need to take some time out and review how they are feeling.
I would advise that she reviews her results with one of her tutors, see a councellor for a chance to off-load and then get away with the family for a fun weekend. Going forward she will need to pace her workload and plan for exams and also plan in one or two extracurricular topics which will relieve the pressure. The phrase I use is 'sharpen the saw'. But it sounds like she has a supportive family around her and thats a really great thing!
Hope this helped


__________________
If this answered your question please click ACCEPT, use reply if there is something else you need to know regarding your question.Positive feedback or bonus is very much appreciated.

Do you think hormones play any part in this disturbed sleep? And yes, she is trying to keep her grades good for college. So many things to consider here. The prednazone stated a side effect of hallucinations, I think, but if she has been off the medication for over a week, could that still be it? How long does that sort of medicine stay in ones system? She does fine with balancing everything, and her mother makes up the rest, meaning if my sister thinks she won't have time for a particular thing, then she will not allow it. Do you think it is rushing into it if she sees a professional right away? Thank you.

No, I think talking to someone now would be a good idea. But I don't think anyone too heavy, a guidance councellor or therapist would be fine. She needs to avoid all stimulants at the moment so cut out caffeine drinks including the fizzy stuff.
The prednazone should be more or less out her system by now, but it will have had a small effect on her natural hormones depending what dose she was on and how long she was on for. After two weeks I would expect to see no residual side effects, but the consequence of the other stresses may take a bit longer to sort out.
Use of a herbal anti-stress tablet might be a good idea and use relaxing music and lavender oil at bedtime so she is completely chilled out by the time her head hits the pillow.
Best wishes

__________________
If this answered your question please click ACCEPT, use reply if there is something else you need to know regarding your question.Positive feedback or bonus is very much appreciated.