How am I wrong about this?

I gave my sister $150 to pay my half of the phone bill which that is the cost for me. She never paid the bill fully and decided to do a payment arrangement instead and only used half of the money for it and used the other half for whatever her heart desired. Now she is asking me basically every 2 weeks for that same amount and the bill is still on a payment arrangement. So it's never the full amount. She makes way more than more so... why am I wrong for refusing it. She keeps saying I owe her because she paid it while I was undergoing cancer treatment. Why do I have to fork over that amount every 2 weeks when I can't afford it

How am I wrong about this?
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