You could use the not function in the XPath. So if animal is the entity and the reference is animal_sibling the XPath is something like:
[not(MyFirstModule.animal_sibling/MyFirstModule.animal)]
This will retrieve all the animals that do not have a self reference.
I do wonder why you choose a multiple multiple reference though. Why should the sibling be able to be attached to multiple animals? One animal can have multiple siblings but the parent should always be one animal imho.
Regards,
Ronald