Can we sell the flat? The one shareholder just dead.

It depends on what type of ownership you hold. Bascially, there are 2 type of ownership for holding the premises, i.e. tenant in common - TIC & joint tenant - JT. You can check the land search to find out which type of ownership you hold. For TIC, each owner can sell their shares to others without consent of other shareholders. Just like the listed company, each sharedholer can sell their shares to others and there is no influence to other shareholders when one of them dead. But if you want to sell the whole unit, it is required to grant the concent with others to reach the total agreement. For JT, one of the owners died does not influence the change of ownership so that it is called 長命契.