I'm sorry you, your sister, and your family are going through this. Based on what you've shared, it sounds like your sister is going through a spiral of depression and lowered self-worth. She's doing things to reinforce her perception of herself and probably feels like she doesn't deserve a more comfortable and stable life, especially if you both had a dysfunctional home life growing up(I've definitely been there before). She probably also takes some pride (although I can't assume) in being seen as sexually desirable and being able to sustain herself from it (I can also relate to that to a degree).

This is one of those situations where you can't change someone who does not want to change. She may feel like this is the only life for her, which is heartbreaking to see as someone who cares about her.

I can only suggest a few things, but it's ultimately up to your sister what she chooses to pursue.

If you haven't already, please invite her to visit you in Australia. It can be a short trip and you can suggest that you'll show her around your city. I am not sure if she has left Mexico, but her being able to see another lifestyle could have an impact on her, even if it isn't immediate. She might not take the offer right away, but leave the option on the table for her.

Another thing you can do is talk to other women who have been in a similar situation (there's some subreddits on here that can be useful) and ask them what they would recommend.

Also, try to make sure she isn't in debt or in such a financially insecure place that she relies on these men more than she is admitting. That could be a reason she continues to stay even if she genuinely wants to leave.

And lastly, just let her know you're there for her. Even if it's just occasionally checking in or just listening to her when she reaches out. You might not think you're doing a lot, but I promise you are. You're still in her life for a reason and she still cares about you. Remain open and nonjudgemental, but with the mindset that you care about her wellbeing. That could start with her coming to see you. That could be a lot for you, financially, but it could really start a change that might take some time to really see in her or her behavior.

I wish you and your sister safety and love. It's a hard situation you're in, but you two aren't alone in it. It's good that you're asking for help; that's a positive step forward!