I want to speak to my sister about my wife's financial concerns for an event that my wife said she couldn't afford to attend
Do you think you have to get involved in this?
Looks like it.
It is not a good idea to tell a sibling what to do. This will lead to conflict in your family.
I have to stand up for my wife for getting in over her head.
Defend your wife, sure, but don't also try to control your sister.