Are Separate Bank Accounts Sinful?

As long as both agree on this arrangement, it is fine, and may even be more practical. Often relationships are strained over the use of money, so separate accounts, as long as they are open to each spouse, can be helpful in avoiding unecessary annoyances. To have a his-and-her budget can be a helpful thing, since it means the couple are communicating about financial matters instead of avoiding the topic and so setting themselves up for a future argument.

The most important thing is trust and open communication, in particular when the matter is something as touchy as material wealth. So this is not a sin, and it may indeed be an meritorious exercise of the virtue of prudence.