You can try to reduce your sister's liability, if possible, by availing of 3000 euro per person per year exemption on gifts. What I mean is the first 3000 euro per year you gift to her is exempt from calculating her CAT liability, but if, for instance, she is married, you can also give her husband 3000 gift that he can pass to her. If you are married, you can give 6000 euro to your spouse, who can then gift 3000 to your sister and 3000 to her husband. If your parents are still alive, you can give them 3000 each, and they can pass it on to their daughter/your sister, etc.
Depending on when your sister needs the money, you can repeat this process next year.
It won't matter much if we are talking about a very large gift, but on more modest ones the saving on CAT can be considerable.