Whether the intent is deliberate or not, the outcome is hurtful and so, yes, I think it is legitimate to forgive. Knowing that the person has a mental illness, and I do not want to venture a speculation about what it is, should make it easier to forgive. We have to keep in mind that even if a personality disorder of some kind is involved here, there still is free will at least to a degree. In other words, even with most mental illnesses the person can choose to express himself or herself in a variety of ways. For some reason, this relative has chosen condemnation and has chosen you. That is forgivable.