In your use case, as you say, simples.
In other use cases with other family structures, not so simples. I will actually find this functionality rather useful, whereas your solution, to me, would be an absolute #fail.
And on a more practical note, if you keep bank login credentials in Lastpass, your bank would treat your approach as having disclosed your PIN to someone else if you are the victim of fraud. The fact that it's your significant other and that you trust her will matter not one whit to them.
Biting the hand that feeds IT
