I have a 5 year old niece, my sister suffers with addiction. My beautiful sister has tried for years to get sober spent 2 years going to counselors and a treatment program successfully completed
her sobriety lasted a few months then her best friend ended her life; my sister relapsed and is often away for days; i take care of my niece who cries herself to sleep some nights, asking where her mom is or why shes not coming back for her? How do i tell her that her mother has an addiction and just canโt stop? Please any help is appreciated 
I want to give you some hope from the other side of this. I'm a mom in recovery, and my addiction took me away from my children too. There were periods of my life when I wasn't able to be the mother they needed because addiction had such a hold on me. I know there were people who probably wondered if I would ever get it together. But I did. Today I'm in recovery, I've rebuilt my life, and I'm able to be present for my children in ways I couldn't be then.
So please don't give up hope for your sister. Relapse doesn't mean that recovery is impossible for her. At the same time, your niece needs stability right now, regardless of where her mom is in her addiction. At five years old, I would keep the explanation very simple: โMommy has something called addiction that makes it really hard for her to make healthy choices right now. This is a grown-up problem. You didn't cause it, you can't fix it, and Mommy's struggles are never because she doesn't love you.โ
And when she cries for her mom, you don't always have to make the sadness go away. Sometimes just hold her and say, โI know you miss Mommy. It's okay to miss her. I'm right here with you.โ Please keep loving that little girl and being a safe, consistent person for her. And keep some hope for he r mom too. Sometimes people really do come back from places that once looked hopeless.
Keep loving her. Keep supporting her. Keep being there for her. Keep showing up for her.
My sister told her children "Aunt Debbie is traveling, trying to feel better..." They were 6, 5 and 2...I was 34. I felt weird at the time, like "honesty is the best..." But, 25 years later, I am sooo grateful for that gentle story, for being able to return (better!) from my travels and for having received the gift of time to heal!
Have you looked into Alynon? Itโs for family and friends of addicts. It definitely has solutions and support there