I don't want to go on my sister's wedding cruise.
Why don't you want to go on the cruise.
It is too expensive for me to go
If you are expected to pay for the expensive ticket on your own then it is understandable that you don't want to go. If your sister isn't willing to pay for your ticket then she should understand.
I will go to the wedding, just not the cruise.
How you spend your money is up to you and being fiscally responsible is commendable. Explain to your sister that you can't afford the cruise and she will understand.