Consider

[Clodfobble]

I'm trying to imagine it from the sister's perspective...

If my mom were living with me as an "only because you're my relative and you're in need" standpoint, and I particularly resented the in-residence of her dog (as I would, not being a dog or cat person)... yeah, I think I could be a little ticked if, on top of it all, I had to pay a vet bill for her stupid dog's antics. But, clearly your sister is okay with dogs, since she was voluntarily watching the neighbor's dog as well.

But you're right, for a bill that relatively low, this is clearly a righteous stand she's making, and not about the money. I'd stay as far away from the whole thing as I could.