Since mlm's are not religion based, they cannot actually be considered a cult, by many times they are so similar in practice and result that it's hard to tell the difference. It is very unlikely that you can do anything to stop your sister, short of going to CA and confronting her. You would have better luck, getting the whole family together and doing an intervention with your dad.
Try and gather as much information about what's going on with your sister. About the mlm, about her financial situation, her living situation, her job, etc. Even if it means hiring a private detective. If your dad feels that he must give her money, then perhaps you can get him to agree to giving her a certain amount and nothing more each month. If you and the rest of your family can get together at one time and sit with him it would be best, XXXXX XXXXX a third party involved like someone who works in law, or therapy that could explain to him how destructive his behavior is toward her even though he thinks he is helping. Sometimes we need a stranger to say something we are not willing to face.
Talk to your dad about helping him get his finances together, perhaps giving you or another family member power of attorney to help. The botXXXXX XXXXXne is legally, there's nothing you can do, as they are both adults. The best thing to try is to present a compelling argument to one or both and see if it helps. Does she know that her father has take n out loans for her? I welcome your thoughts, let me know if you want to talk more
Do you belong to a church? Whats your dad's city/state?
Understandable. While I do a little looking around in that area, I noticed you said parents, I guess I assumed your mother wasn't around. What's her opinion of all of this?
I don't personally know someone in that area, and I would just pick a name and send it to you if I don't know the person and possibly make matters worse. I did a search and came up with this list. I would go down the list and contact each person and ask them if they have counselors familiar with this type of situation. People who are familiar with addiction are the type of people you want to talk to. You can also contact the organizations here, here, here and here. Get a notebook, and for each organization you contact, give them a page. On that page you'll want to write who you spoke to, any references they give you, costs, location, phone numbers etc. Once you go over all of the people you called you can make choices based on how they helped you, what the costs are, etc
When a parent goes so far above and beyond for a child, there's usually a fair amount of guilt involved about something. The parent usually feeling like they didn't do something right, they did something wrong or they didn't do enough.He may need to get into therapy, but many times this isn't something that can accomplished....it's worth a try, if you think he would consider it.
Here's a few more links about MLM and how they coincide with 'cults' or a cultish way.
I welcome your thoughts, let me know if you want to talk more