Couples like you hardly give this any thought until someone brings up the issues like your older sister has. We do not think separate money means you do not trust each other. It is more likely this is just more convenient. But the issue does bring up one thought. We think it is important to have some because it does give something tangible to share equally in other than children.
Marriage is a partnership with a contract.
If you share nothing, the contract has less meaning and you could be cheating yourself unknowingly. In marriages we think it is important to declare what you are keeping separate and what you are joining. The important thing is that you and your partner agree on what those separate and joint things are. For many couples the largest single tangible joint financial venture is their home. Down the road spouses often add a retirement portfolio which goes beyond the ones they each have at work.