My sister won't listen to me.

My sister is mentally handicapped. She has the mentality of an 11 year old in an overweight 40-something old woman.
At times she will get stuck on something and can't let it go and it becomes impossible to talk to her about anything. This leaks over into the rest of the time which causes her to just not listen to people or ignore what they say if it is not what she wants to hear. It is very frustrating.
I don't want to nag her about things but it is the only way to get something in to her brain at times because she may ignore or just not hear what I am saying the first twenty times. but after a while it finally makes an impression and she starts hearing what I am saying and usually starts doing what is needed.
It is during the immediate times when she is having some problem and is stuck on something in her head and also won't listen that it is the most frustrating. I try to help but she won't listen and wont do what is needed to get past whatever the problem is and it becomes impossible to get anything done with her.
One of these type things happened today and it caused me to go into a near anxiety attack because she would not listen and would not stop sying the same things over and over and so we could not get a simple thing done. I had to just stop and not talk to her and not be around her or I would have gone crazy.
I'm typing this to try and expunge it from my own head and relax but so far it isn't working. Oh well, thanks for reading anyone that is. I hope you have a better day than I am having.