Thank you to everyone who provided advice. It was all very helpful.
I called the director back and asked a lot of questions. They wanted me to commit to the full 12 weeks and the sessions are large group sessions with 10-12 'clients' and one family member per 'client'. There's a lot of education on addiction and rehabilitation as a part of the sessions. My sister has been in treatment since last August and 'is doing well' according to the director.
I made the decision that I am not able to commmit 4 hours per week (including drive time, dinner, and session) to my sister to help her with her recovery. I can't do the time commitment, nor can I do it emotionally. Instead I asked if I could get my sister's contact information to initiate communication again. They agreed this could work, as my sister is not allowed direct phone calls out to anyone. It may help her to at least hear an update on C.
After calling them, my cousins called me and asked me to testify in April for the custody court hearing. My sister has not only made direct contact with them (which I don't think she is supposed to) but is taking them to court to ask the judge to modify the custody agreement to gain more parental rights in hopes of eventually gaining full custoday of Baby L.
Now I'm questioning my decision on even writing her a letter. I think my instincts were right that she is getting better in an attempt to get Baby L back (this is a common pattern with her). At any rate, I will definitely testify again.
Re: Update & My Decision
PAL/PGAL Welcome
Wow...I think you definitely made the right decision to not go to the sessions. As much as I'm guessing you want your sister to get help and get better, it doesn't sound like it would be in you/your daughter's best interest right now.
Maybe you should hold off on the letter until after you find out what happens with the custody hearing?
Oh, Sally, I'm so sorry that it seems your sister really is following through only because she wants to regain custody. You did mention that that's a main goal of the program, though, so I guess it is to be expected. Maybe the best-case scenario will work out and she will stay clean long enough that it actually takes.
I am so sorry you are dealing with this right now. I can only imagine how you must feel torn. While it's your sister, protecting your family (and extended family) is more important.

Go with your gut and don't let yourself second guess things... it will drive you mad