Thanks for your question. It is possible that a woman can feel or act differently (in this case, better) while pregnant. It could be just as you mentioned; the hormones contribute to the change in personality or behavior.
Generally, people with schizoaffective disorder have difficulty handling stress. Pregnant females with a schizoaffective diagnosis are at a greater risk for psychiatric complications during and after pregnancy. How well your sister will handle the stress associated with pregnancy is difficult to know. The truth is only time will tell.
There are plenty of cases in which women diagnosed with schizoaffective disorder remain stable during pregnancy. But there are also mothers-to-be heavily impacted by the stress of a baby, to the point where psychosis during pregnancy develops, or a postpartum depression or psychosis erupts after the baby is born.
The fact that your sister has already had a psychotic break does mean she is at an increased risk for a future break. You said she also stops taking her medication because she thinks she is better. Medication noncompliance can and often does lead to future psychotic breaks. It is very common for people with schizoaffective disorder or schizophrenia to stop their medication because they “feel” better. It is important, however, that she’s encouraged to take the medication because taking it consistently is one of the most effective methods of preventing future psychotic episodes.
I also want to point out the fact that your sister was diagnosed with schizoaffective disorder and this likely means that she has had other psychotic episodes. A schizoaffective/schizophrenia diagnosis can technically only be given if there has been more than one psychotic break.
You asked whether your sister really has schizoaffective disorder. I cannot know for sure but the fact that she was diagnosed means that it’s at least a likely possibility. Also, you wrote that she “makes irrational choices, never understanding consequences and seems to lack sympathy for anyone but that is how she has always been.” These are some of the hallmarks of schizoaffective/schizophrenia disorder.
Please keep in mind, however, that I could never make an accurate diagnosis over the Internet.
As for your role, try to be very supportive and also encourage your sister to stay in contact with her doctors and therapists. Most critically, and a point that I cannot stress enough, she needs to take her medication consistently, not just when she feels like it. Medication is essential in the prevention of future psychotic breaks. If she has help and support from you and other family members, and stays on her medications, there is a very good chance she can remain stable. Please write back if you have any further questions.