My DH says she was never taught to reason things through, so she's just living day to day, clueless as to how the consequences of her actions affect herself and others. How two siblings could be so different, I'll never know.
Honestly some people just do not have that "Executive Function" working all the way...the ability to plan and for-see consequences.
You may need to do that for her if you choose to interact with her.
So let's say you want to buy her some food. It would seem that frozen meals may be the way to go...they don't spoil, they are easy to make, and there are "Healthy Choices" one could buy. So you say "I will buy you some frozen dinners, which of these do you want?"
Or if she is doing something like shaving "Don't shave in my car. That is for the privacy of your own bathroom."
or "Don't drink my beverages. Adults don't share drinks unless they are married. Bring your own or I will buy you one."
You are assuming she is a fully functioning adult, but she is showing you she isn't. Be more direct with her., but still kind.