They do have strengths they never knew they had, and they can meet situations they never anticipated. A good start while learning to cope is to learn as much as possible about mental illness by reading and talking with other families. NAMI has books, pamphlets, fact sheets, and tapes available about different illnesses, treatments, and issues you may have to deal with. No-one is to blame. The mental illnesses are not on a continuum with mental health.
Mental disorders affect more than the afflicted.
Despite your best efforts, symptoms may get worse, or they may improve. If you feel extreme resentment, you are giving too much. It is as hard for the parent or sibling to accept the disorder as it is for other family members. Acceptance of the disorder by all concerned may be helpful, but not necessary. A delusion is not amenable to reason, so it needs no discussion.