I have been involved in the drama of my 28 year old son's life for the past 10 years. It is definitely a process to free ourselves from their drama and the guilt associated with the life they chose, but blame you for. My daughter has her boundaries in place when it comes to dealing with him. Smart girl. I've learned a thing or two from her. When there are firm boundaries established the addict gives up and moves on to someone else to fill their needs. He doesn't bother her anymore.
I began to educate myself about addiction 3 years ago. After a miserable holiday (AH related) last year I committed myself to attending Al-Anon regularly in addition to reading daily here on SR. It has changed my life. His attempts to manipulate me and to make me feel guilty are not working for him like they did in the past.
I can't say I am 100% past the guilt, but I have more peace than I've ever had concerning him. This quote has helped me put things into perspective when I begin to feel the burden of his poor choices.
"Our background and circumstances may influence who we are, but we are responsible for who we become".
I would encourage your daughter to educate herself about boundaries. Al-Anon is a great place. Her priority should be her young son and without healthy boundaries set in place for her brother she could end up on the never ending merry go round. Best of luck to your family.