I'll keep it as short as I can. (Throwaway, obviously)

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My sister has owed me money since Christmas. We had agreed that we would buy Christmas presents together a couple of months beforehand, but about a month before Christmas she tells me she won't be able to pay for the Christmas presents. We had only bought a couple, but we agreed that I would buy the rest of the presents using my own money, and she would pay me the £220 back over the course of January through to April.

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However, we're now into March and I haven't seen a single penny of the money she owes me. She's told me that she had some unexpected bills to pay off, and while I could understand that, she says this every time she owes me money, and it's been frustrating me a lot. On top of that, I know that she's went out for drinks with her friends on at least 1 occasion, though I don't hold that against her.

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So on to the babysitting, I had agreed to babysit for her on the 9th this month, which is a Saturday. I know that she's going to a wedding for one of her boyfriend's family members, and that she's looking forward to going, but the lack of any form of repayment has been weighing on my mind. I'm at a bit of a loss for what to do, so I figured I'd put it to you guys. WIBTA if I told her I wasn't going to babysit for her due to the lack of repayment, and if so, what would be the better way of going about this?