Sibling magic's are magic that are closely related and are easy to learn for those who already have a magic some examples:

Earth and plant

Shadow, necromancy, death and nightmare

Light, life and dream

Shadow mage's often prefer forests, cities and rainy areas than sunny and flat ones

Nightmare magic is a form of illusion magic that the user has very limited control over such as the person it shows to. The actual illusions are decided by the target persons nightmares.

Shadow magic is also known as soul magic and shadow mage's are very often known as soul eaters

The shadow realm is a place where as shadow travelling is where a shadow magic user teleports to a place where a sufficient amount of shadows

The more powerful the shadow mage is the less shadows is needed for shadow travelling

A shadow mage's shadow is a small realm where only the user can access it and send things into it