Don't want to be unkind,but....

Maybe you might have another conversation with her sister before she moves away. She may think you will be taking over caring for her sister when you are clearly not.

Reply to a message with something like "I wish you the best but I cannot be your friend because it's not a healthy friendship" and then stop answering completely.

You don't owe her any further explanation

Do not answer the door either, if there is no peephole, you could always get a ring camera installed so you can see who is there. Also ask whoever lets her in to stop doing that.

It's a very hard thing to do but it will be worth it, I promise

Some people are so thick skinned and entitled that you have to be very blunt with them. This woman has a relationship with you. She knows your weaknesses and is prepared to exploit them.

From a psychological perspective Ive often found that getting a "third person" involved does the trick. Third party intervention is a classic method for restoring the power balance into a situation which has got out of hand. The problem person has no relationship with that individual who suddenly rocks up and catches them on the back foot. When youve tried every other trick you have to play hardball.

They dont have to be aggressive. Just explain their concern that you are "upsetting my aunt/cousin/friend by doing X and I will not allow this to continue. Any further contact must be through me."

If you're that boring why does she bother?
Supposedly, me thinx, because everyone else has had the gall to be upfront and rude to her so that she understands she isn't welcome at theirs...even her own sister is moving away! 🤣
Well, you don't have to be rude but you definitely need to reinforce that you are busy with your own life and she's somewhat in the way!
Or, every time she comes round give her some cleaning supplies and start her off in the bathroom, tell her your toilet needs a good scrubbing! Lol