The diamonds were not really stolen. It was also a prank by the sisters.
On the blind spot, Yuri trampled. The briefcase was exchanged with the empty one in the shelf.
When Sato, picked it up, it was heavy as the diamonds because of the dry ice.
The reason why Yuri didn't took the briefcase down it is because the dry ice haven't melted yet.
the note says to go to Beika Central Park to have a time to make the murder.
The first time the lights went out. Miki exchanged herself to a mannequin.
Miki met Yuri at the rooftop and strangled Yuri to death breaking her necklace.
The second time the lights went out, Miki exchanged with the mannequin.
A piece of pearl stucked on the hem of Miki's pants
The pearl was hidden by Ryosuke to protect the family.
Conclusion
Shinichi reveals that the murderer is Miki Noguchi. Miki said that the pearl was not luckily got into her hem but was put by Yuri. Miki, after dropping the pearl to the floor, Shinichi realized the place of the hideout of the stolen diamonds.
Motive
The reason why Miki killed Yuri is because she wanted to surpass her sister.
