Here's a normal conversation. ...
Me...when will we be able to move the desk into your room so you can study in peace? (It's been 4 weeks)She uses the dining table.
Sis...can't do it now, too many other things to do. It's on my list.
How about saying something like:
You: We need to move the desk into your room. Let's get it done on Saturday (or pick a day).
Her: No, I am busy
You: I understand you are busy and have a lot going on. I need to get this done. If we can't do it on Saturday, we can do it either on Monday or Wednesday. Which do you prefer?
Don't give her an open time frame. It will never get done. It is your hose and while she is welcome there, this arrangement has to work for you.
Me...No I don't want to watch that now, I'm watching television.
Sis...throws her phone on the table and storms out.
If she throws her phone on the table and storms out, let her. It is not the best response for her to have but it is what she does. Don't accept or accommodate such poor behavior. No response from you is required. In fact, any response, other than leaving her be to self soothe could result in reinforcing her behavior.
Next time it happens that she wants to watch something you can say I am already watching this and would like to finish it. We can watch ___ next.
Stay calm, allow her to have her reactions without internalizing them. This will take practice on your part though. Over time, things can improve for you as you begin to detach emotionally. Don't let her emotional state dictate your internal state.
Can you see how this can apply to any other situations with her?
We do have some good times of course, and we have laughed until our sides hurt.
This is great to hear.