Description:
A strange phenomenon that can only be achieved by siblings (mainly sister to brother) who are very close, when in a time of need and their bonds are pushed to the limit one of the siblings (usually the girl but very rarely the boy) takes on some of the other siblings form and their powers are increased. It only happens once every century and thus is very rare, it can only be achieved if one of your relatives wasn't of the world you live in.

Only known cases:
Anthony Zane Jaeger - Shard Sota Jaeger
(Date 14/11/2015)

When Anthony and his lover Kevin were under a spell set by their hated enemy Springtrap, they attacked Shard, her friend Lucy and Kevin's sister Amy. Shard was overcome with grief for her brother and almost gave up on living but through Lucy and Amy's encouragement she broke the barrier and the Sibling Bond dosed her with enough power to snap Anthony and Kevin out the brainwashing also destroying the evil sol outfits they were wearing. Sibling Bond Shard is the most powerful of the three, her immense power flows from both the music she plays through her headphones and the strength of her friends.

Brian Harmon - Lucy Naro Harmon
(Date 13/7/1987)

Not much is known but there were reports of a giant in the area... (Hint hint Lucymidnight <(OvO) ) When in Sibling Bond mode she becomes extremely clever and analysis her opponent before attacking, very skilful and strategic.

Kevin Rodney - Amy Rodney
(Date 14/11/2015)

Alongside Shard and Lucy she helped destroy the evil sol outfits and snap Anthony and Kevin out their brainwashing, Amy is the fastest out of the three and the most agile.

How does this happen?:

No one knows...

What are its effects?:

Listed in the known cases, the effects vary from person to person and it depends on how strong the bond is and how close the siblings are.