Sorry I didn't mean that because your DC were private you should pay for your sister. BUt it would have been even more shocking if you didn't even pay for your own DC and he asked.
I would have been less than impressed had my parents asked me to fund my (much) younger sisters private education when I went to state school, unless of course I had loads of money and wouldn't notice the expense.
Hpoe you manage to get hold of someone about the weekend away.