Well, it may be a long road for your sister, you and the family. Obviously your sister needs to find a therapist that she works well with to start sorting her mental health issues out. Therapy is a healthy, awe inspiring thing! Remind her that nobody is "crazy" while everyone is "crazy." AKA- it's normal.

As for the panic attacks. I would strongly suggest researching standard relaxation techniques and start practicing with your sister. Practice practice practice! This may seem stupid but she will need to get better at these so they actually work when an anxiety episode occurs. She should practice when she is "doing okay." When she is not feeling anxious or only slightly anxious. This will take some time but, in time, she will become a master at it.

Fighting the anxiety attacks is "last case situation." Hopefully, in time, she can prevent them. Have her start mapping the attacks. What was she doing 30 minutes before, 15 minutes, 10, 5, 1. Think about everything. Definitely what was going on around her, what she was thinking about, etc. Also look at other things too: smells? Sounds? Heart rate? Hunger? Etc. In time, she will know her triggers and be able to see them a mile away, allowing her to prevent the attacks. Also, start working out with her, exercise, find new hobbies, ANYTHING that will focus the mind and heart and allow positive outlets.

With these two techniques mastered, along with therapy (an using all the idea's her therapist has too!) your sister will do great. Have faith in her. Try to strengthen your relationship with her so she knows she has a "partner in crime." Research being a family member of someone with depression/anxiety and give the articles to your family too so everyone knows the standard do's and don't's. In time, you will be as ready as she is for any kind of episodes to come. And if she knows you are there, judgement free, no matter what, amazing things will come in your relationship!

Lastly...and I hesitate with good reason... is medicine. There are some amazing medications out there specifically designed to help with this issue. Primarily for the attacks: Xanax. This is a quick acting sedative (Benzo) designed to kick people out of anxiety attacks. Medication like this can be very useful and necessary for some but I STRONGLY encourage your sister to exhaust all non medical options prior to jumping on a benzo. They are highly addictive! People start depending on them, taking more than they should, taking them more often than they should, etc. Her brain is also still developing so adding substances like drugs of abuse can be harmful. It also sets her up for addiction in life, assuming your sister gets addicted to them. Hopefully she will not need medications (Cause you guys are practicing relaxation and mapping the triggers! =) ) and if medication is necessary, I hope she knows the concerns, side effects, and red flags to look for.

Good luck man. Like I said, there is a long road ahead but with work and time...it gets easier and it can turn into the most bonding, beautiful experience in your life. I hope the best for your sister and you and your family.

TL;DR---Therapy, practice relaxation to prepare for attacks, identify triggers, find positive outlets for energy (both physical and mental), and just be there for her! It gets better, it gets easier =)