An otherwise good friend always took advantage of the 'split the bill' rule because, as a non-driver, he always had a lot to drink and one of us always ran him home. He only ever brought a credit card and often took all the contributions, including tips, and paid only the bill on his card! Last time we all agreed to pay only our portions of the bill, with equal tip each, and insisted the tip was added to the receipt. Amazingly this time he managed to find cash. He also had to get the bus home. A bit hard, but after years of this behaviour we'd had enough. He's still talking to us, but we'll see what happens next time. I think you need to find a way of telling your SiL enough is enough!
Static caravan purchase is it worth buying