I assume your sister has seen an otolaryngologist (ear, nose and throat doctor). If not that would probably be the first step though we all know that ENTs are not that helpful with regular tinnitus. Since your sister's tinnitus is due to an accident which I'm guessing may have damaged her ear drum, then there may be something an ENT can do. If the injury and tinnitus are relatively new, then she might want to look into hyperbaric oxygen therapy (HBOT) or perhaps some steroid treatments, depending on what the injury actually did. HBOT in particular has been shown to be helpful in cases where the inner ear hair cells have been damaged from sound trauma. The extra oxygen helps keep the cells from dying off so that they may eventually recover. Talking magnesium and zinc have proven to be somewhat helpful to reducing the volume of tinnitus. No cures, but many things can help reduce the annoyance. Different things work for different people, however, so it is never easy to figure what is the best for anyone in particular. Its just trial and error.
If the tinnitus is due to a significant hearing loss then hearing aids can definitely help. An ENT and audiologist can help there. Of course, no one looks forward to wearing hearing aids, but any relief from the extreme annoyance can improves one's outlook immensely and that can count for a lot even if they hate the idea of wearing hearing aids.
After that I would suggest masking therapy. While it doesn't do anything to correct the problem, it at least makes it more bearable which may help get her out of the funk that it sounds like she is in. There are lots of different kind of masking aids - anything from dedicated sound machines that make white noise, brown noise, pink noise sounds, as well as water and wind sounds, to CDs and MP3s with nature sounds (birds, rain, crickets,...). I myself found these MP3s very helpful:
http://mytinnitusmasking.com/downloads-2/index.html. Also these:
http://simplynoise.com/. Just make sure if she uses earbuds or headphones to keep the volume low. The volume should be just loud enough to barely cover the tinnitus sounds. Another thing she can do if she is having trouble sleeping is to listen to soothing music or perhpas an audio book. Many people find it difficult to read when they first get T, but most don't have a problem listening to an audo book.
Next up would be just being there for her to support and encourage her to do her best to ignore the annoyance and keep busy. Most people habituate to their tinnitus with time. The time for habitation varies, however, and it may actually take a couple of years. Most people, however, become used to the sound before then. As someone who has only recently got to the point after nearly 7 months where I can now go for a few hours without the sound bothering me, I remember very well how incredibly derpressing and anxiety producing it is to not be able to find the "off switch" for the incessant ringing. At the start I simply did not believe that I could ever get used to the noise, but with time I have (though it still bothers me more than a few times a day). Providing encouraging for her to hang on and believe it will get better with time is one of the best things you can do.
If your sister is experiencing great anxiety from her tinnitus, then she may want to see a doctor about taking an SSRI, or antianxiety medication. I would not recommend any benzodiazepines unless the anxiety is out of control, but many people find great relief through them. Benzos are tricky and can lead to other problems particularly when it comes time to withdraw from them. Pregabalin (Lyrica) is another drug that some have found helpful, and it is not addictive like benzos.
Lastly, I would encourage her to join Tinnitus Talk herself. There is a lot of good information in this forum, and plenty of people who have been through it all, or who are still in the midst of it, who are willing and wanting to lend help and encouragement. There is quite a bit going on research-wise with tinnitus, and just keeping up-to-date with that by plugging into this forum and becoming aware that a successful treatment may not be that far away can be comforting.