My brother used to hit me a lot, but my parent justifies it by his autism and schizophrenia. Was it still abuse?

By enabling the behavior, your parents are not advocating for your needs. Regardless of a mental health diagnosis, the behavior is not appropriate or acceptable. The parents have a role in providing a safe and nurturing environment for all the children under the roof. A large part of working with diagnoses such as autism and schizophrenia is by following a care plan established by a mental health provider. These types of plans can assist parents and family members towards correcting behavior while still having a safe living situation for all family members. Abuse can persist in many ways- emotionally, physically, financially, etc. Without support from your parents validating your needs and safety, it presents a harmful environment.