My sister wouldn't pay either on account of the fact she hates my guts and worships money.I don't think you should expect your sister to pay but I understand you maybe wishing she would!I don't think it is right though to take money from your sister for your own care,not if she has a husband and family of her own.On the other hand if she has a lot of money and it is for something as sensitive as your teeth then I think it is ok to expect her to help.It also depends on how close you are as sisters and if yours is a loving relationship or not?