How do you deal with a family member who constantly lies and won't admit to it even when caught with hard evidence?

I have a family member who lies a lot. Recently we went in on a gift for our parents anniversary and this person was in charge of all the details. I was told it was all taken care of. Now that my parents are ready to go on their trip in a few months, there is suddenly a big problem with the arranagements and they may not be able to go on the trip. I found out hard evidence that I was lied to and I called them on it but they somehow made it out that I was being unreasonable. How do I deal with this?