Sorry you've been having a tough time with a family member. I did not read your other threads so I'll focus on what you shared here. In general, if you don't want to do something, I would recommend keeping it simple such as: "I don't have that information" or "I am not able to do that" or "I don't have time to discuss this." You don't even need to give an explanation.

If she pushes once, you can repeat your original statement. If she pushes twice, you can simply say "I don't need to discuss further. I'm going now." And leave it there. If you hold your ground, she will learn to stop asking. Not right away, but in time. The difficulty is that you were previously unwittingly reinforcing her behavior (I realize you were trying to help) of continually going to you for help. When you withdraw the stimulus she's seeking (help) she will no longer be reinforced and will stop seeking the same stimulus from you.

I believe you will find more power and peace by not speculating about her internal state or motivation. We can't really know what's going on in someone else's mind or soul. You have a right to withdraw your help so you can do so and quietly wish her peace. I wouldn't recommend a phrase like "you're on your own" because that could sound judgmental or a bit harsh. The sooner we set boundaries with people, the less risk of building frustration and lashing out. I'm not suggesting that you would lash out, just that it can be an unfortunate side-effect of not setting boundaries with others and then resenting them when in many cases they may be oblivious rather than intentionally trying to bother us.

Your question reminded me of a situation I had a long time ago with a friend. I was getting ready to go to an event being held in my honor. I had a long day at work and needed to tie up a few loose ends before going to the event. My friend kept texting me questions about the eve. What are you wearing? etc. So nothing urgent. On another occasion, I would have gladly answered her Qs but I was busy and stressed and needed to get off my phone in order to complete my transition from work to the event. I sent the following message: "I can't keep texting now love. The more I'm on my phone, the less time I have to get ready and be there on time. I'll see you there soon."
She replied: "K." I got a vibe but left it there. Then I showed up to the group gathering in my honor and the friend I mentioned was sulking with me. Wouldn't make eye contact or talk to me. I tried to clear the air (when others couldn't hear) and said I'm sorry if my text sounded a bit abrupt, I was just in a rush. She continued to sulk all night. I did my best to focus on the other friendly and kind people there. But I confess that she did spoil my evening a bit. I don't think I did anything wrong but apparently she thought so.