Providing a diagnosis over the Internet is impossible. Diagnosing involves interviewing an individual, in person, and gathering a great deal of information about their symptoms, personal life, and so forth. Therefore, I can only provide a general opinion and not confirm nor dispute a diagnosis.
The fact that your sister has been diagnosed with a disorder would suggest that she has been assessed (in person) by mental health professionals. They likely thoroughly evaluated and observed her symptoms, and subsequently arrived at a diagnosis of schizophrenia. Diagnosing mental illness is not an exact science and thus, if she were interviewed by multiple clinicians, she may receive different diagnoses.
Nevertheless, schizophrenia is the disorder that they thought most matched her symptoms.
Bipolar disorder and schizophrenia share common symptoms but they are different disorders. The main difference between the two is the mood component and mania. If an individual is experiencing both symptoms of schizophrenia and bipolar disorder, they might receive the diagnosis of schizoaffective disorder.
You also wrote that she doesn’t have delusions, hallucinations, disorganized speech or catatonic behavior. Maybe she is not currently experiencing them but perhaps had them previously. To be diagnosed with schizophrenia, she would have needed to display at least one of those symptoms.
I would encourage you to contact your sister’s treatment providers, about your concerns, if possible. They might be able to explain why they thought that she has schizophrenia and not bipolar disorder. Arguably, what’s most important is whether or not her treatment is working and if her symptoms have improved. If so, that is encouraging and evidenced that she is on the right track. If not, then she might need to return to her treatment providers to discuss a modification or seek out different providers for a second or third opinion. Best of luck to both you and your sister. Please take care.