As above, moving 15k to a personal account esp when its way more than you usually have will set off a lot of alarms. You'll just need to talk to them, you and possibly your sister will likely need to provide some info as to where the money came from. They're a lot more cautious with transfers than they used to be, I tried to repay a friend just over 1k a while back and they went mental, account locked down & fraud department on the phone etc, and in the end they still refused to make the payment, no matter how long I said I knew the guy or that the transaction was genuine and I accepted full responsibility *shrug*
(Normally when I need to make a large transaction I'll talk to my bank beforehand, so they can either expect the payment or advise me if it will be an issue)