This is a guide that walk you through Sinon’s Hollow Event.
Sinon Hollow Event
Bird nests are found near Bastea Gate’s Floating Ruin tower
Teleport to Floating Ruin Plaza at Bastea then go to the north exit to enter Corridor of Negotiation
Then go to the east exit to enter Bastea Corridor of Drills. Then go to the exit to enter Garden for Lovers to Rest
From there, go to the north area and there is a small area which you will see a bird nest and receive a key item
Then go back to Agil’s shop for an event. After that party with Sinon and go back to the bird nest area
A strange plant can be found in Arlebast’s Outer World Area
Alone go to the Queen’s Chamber. Then go southwest of the map and you will find many heart shape thing in the area
Go back to Aincrad and go to Agil’s shop for an event. Then party with Sinon and go back to the heart shape area
Then prepare to fight five NM LV115 Gigantic Plant.
After that go back to Agil’s shop alone for an event.
Sinon Hollow Event End
Source: http://www.gamefaqs.com/boards/734608-sword-art-online-hollow-fragment/69647786 by Sunsetdeska