If she doesn't want to listen to you, you can't force her to. If she's an adult she should know better, and if everyone keeps baby-ing her she'll never learn. I don't know your situation, but the best thing you can do is just make her aware of the situation and ask her to think about how to take her own course of action that'll be best for her in the long run.
@ Your blog: To be honest.. this rant is kinda confusing so I really don't have any idea what you're talking about either. But if someone around you is older than 18, they have the right to do with they want with their money, legally, and they really -don't- have to listen to you. However, they will have to deal with it when it gets ugly, not you, and hopefully they'll learn next time. Focus on yourself and what you need to do to succeed in life.