Lucie,
That's where "boundries" come in. You need to work on you.
Focus on you. A recovery program for codependents helps learn to
deal with those types of situations. We all want good relations with the
people we care about. With addicts, it's alot harder. A rollercoaster, as you put it. Henry Cloud writes a book called, "Boundries." It helped me alot when dealing with my son. Also, "The Enabler: When Helping Hurts The Ones You Love" by Angelyn Miller.
I know you love your sister. I have had many family members with addictions and it took my son's addiction to set me on a good path of recovery for me.
Time and recovery. It will get better.
Keep comin' back.
Peace and prayers,