Thanks everyone. I really appreciate you taking the time to reply. Like I said, Im so out of my depth but I feel like I cant just do nothing.

To be clear, I dont want to take away her rights to her money. I know its hers to do with as she pleases. I just want to be able to stop her ending up where she started a few months down the track. When she sold her house she recieved just under 100k dollars. Itwas gone in 6 months. She has told me that she is inheriting 500k from her parents. It is clear that no amount of talk from me or anyone else gets through to her. If she doesnt agree with what I say to her, she will pretend she does and still do the opposite.

She hasnt been diagnosed with anything other than depression as far as I am aware. She also has a thyroid issue which she claims affects her decision making ability. She obviously has something more serious going on, than depression. My dad believes it may be aspergers or something similar. He says that its like she is stuck at 14 years old.
It is so hard to describe her to people. Unless they have spent a moderate amount of time with her, people dont seem to get it.

She is a danger to herself financially. She is already talking about all the things she wants to buy me. Everytime we mention, we'd like to do this or that for DS its, "well when I have my money Ill pay for it!"
Socially she has no idea. She is intelligent. Just not when it comes to social or financial decisions. She doesnt understand sarcasm, tell her a joke and she thinks you are serious no matter how out there the statement is. She doesnt understand about social hints and cues. Ie, she will come over in the morning, hang around all day then if we say we have to go out she wants to come too. If you tell her your tired she offers to look after DS while I have a sleep. If you tell her you have housework to do she offers to help. She doesnt get that its a hint for her to leave. If you come straight out and say I want you to go you hate her and never want to see her again. Sure, its nice that she wants to help, but after talking to her (which is hard work in itself) for 6 or 8 hours, it becomes exhausting.
The time she is here, or with anyone else, its not limited to me. She spends the conversation talking at you not to you. She doesnt actually listen to anything you say.
Despite her visiting from 9am to 3:30pm one day a week, she wanted to make it 2 days a week. She cant understand why I dont want to spend all my spare time with her. Dont I like her?

It all seems relatively normal when typed out, but its not. Its exhausting. And frustrating. If it wasnt for the fact that I am her only family and her daughter, I would have bailed years ago. Some days I seriously consider it, but ultimately, Im all she has and I cant do that. Its not her fault she is the way she is.

Im not exactly well off myself. I dont know that I can afford to see a solicitor. I was thinking of talking to a Centrelink social worker and seeing what they can do to help me, help her.

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Ill talk to her about doing a will too. Thank you for that suggestion. DF said last night that he thinks the state takes everything in her estate unless she has nominated someone in a will. Is that right or does it automatically go to us (my brother and I)? I just assumed it went to the next of kin unless otherwise specified in a will.

My plan with buying a house, if she agrees, is to go with her and hopefully steer her towards a good decision. She has always said she needs a 3 bedroom house. But that will be the majority of what she inherits. She is on her own, and as much as she hopes to convince my brother to move back home, I doubt that he will, and he shouldnt anyway. He is 20 but she treats him like a 10 year old. So really she only needs a one or 2 bedroom place. We house hunted with MIL for months so I can tell her we have a reasonable idea what to look for. So that shuldnt be an issue. And Im hoping that once she doesnt have to worry about paying rent she will be better off. Feel more secure, and more income left to buy food, clothes etc.

Sorry its a bit rambly. Just trying to give you a better picture of the situation, but its complicated and I find it difficult to articulate.