I would try to make an appointment with a psychiatrist, and have you or someone else she trusts go with her the first time or two so that the doc can hear outside observations, since your sister is probably not aware of how she is acting.
Many psychiatric illnesses are very hereditary, so it is quite likely she has an illness such as your mothers' and brothers'. Keep an eye on her and try your best to get her to see a doc. If your bro or mom are on medications that are helping them, get a list of what they are taking and bring it along when you go to the doc. Often if a med works for one family member it will work for another.
This is very random its like you send your very enegetic, talkative child to school and when they come home from school or maybe even a week later they changed drastically theyve become paranoid, not comphensive, and not capable of making simple decisons. Im not sure whats going on with my sis Ive alerted family members and trying to set up an appointment with her primary doc. She did start taking some prescribed pills Birth control she says, she also have a couple of male friends so there is a bit of drama, she also turned 21 in march and has been going to clubs and hanging out, as far as I know she doesnt do drugs she a good kid. Another change in her she use to dress up she the girly girl so you never see her dress down so it was a shock to see her where tennis shoes and a pair of jeans. It seems as though someone else took over her. She walks around like a paranoid zombie sometimes I start too see my ol sister but then she goes back to being weird again, and she doesnt live with me she just visit. Everyone close to her notice the change and are scared. Everyone has a theory one says it could of been something traumatic like a gang rape, or she experience some sexual abuse from someone she knows, or drugs , or someone slipped something in her drink maybe..( it shouldnt of lastes this long if it was that), or just stress overload... but what I am saying the way shes acting is 180* degree complete change THIS ISNT MY SISTER. ok examples of some incidents When she was in the car in the backseat we were making a left turn she mumbles " dont turn left no left turn left is bad right is right" .. I looked back like what the hell. another time she asked for a pen and the pen I gace to her happened to be red and she give me a mean look and I asked whats wrong she says " red is evil give me a white pen" another incident we were outside walking into a food joint and she freaks out saying " theres too amny people we have to leave too many people" and she drags me out!! another incident we were sitting in the room by the computer and the screen blacked out from the time out and she screams stop as she ran out the room into the bathroom. I asked whats wrong and she gets mad she thought I turn the computer light out and I explain to her it did it by itself. shes all of a sudden afraid of the dark. SHE WAS NEVER LIKE THIS. shes afraid I would leave her side when Im with her even when she goes to use the bathroom she peeks out the door to make sure im still there.
Sounds like she got the schizophrenia side of it maybe since you didn't describe any mood episodes, but you never know. It could also be bipolar disorder with psychotic features or schizoaffective disorder, among other things. I would say the schizophrenia or schizoaffective disorder is likely since it runs in your family but again I'm only a consumer and not a professional, it would be best if she was taken to a doctor. This is not the end of the world however, all of these diseases I mentioned are highly treatable under most circumstances. A good example is myself; I have had schizophrenia since I was a kid and while I could be doing A LOT BETTER than I am, I at least have a job and function half assed in society and social affairs and such.
It sounds like your sister needs to get to a psychiatrist. She is at the age when bipolar is often diagnosed for the first time. If she becomes really paranoid or paranoid to the point of becoming violent towards herself or others take her to the hospital. She will likely hate you at the time, and perhaps for a short bit afterwards, but once the meds take effect she will likely see it differently.
Take care best you can and make sure you are taking good care of yourself through this.
Mood and emotional disorders can strike at any age and since it is in the family already it is very possible that she has developed a problem. The best thing you can do is to see a psychiatrist and get her on medication and perhaps a short hospital stay until she is stable. This will prevent any self harm. I am bipolar and it hit me at 31 so I know how devastating it can be.
You're right to be concerned about your sister's sudden change. It could be due to a variety of things too numerous to consider here. If you can persuade her to see a doctor, that might help shed some light on the cause. As her sister, you may be better positioned than any to lovingly convince her to either confide in you, or to have her change checked out. You mention your mother suffers from "bipolar schizophrenia", which I take it probably means schozoaffective disorder, which is basically a combination of the two syndromes. There is a genetic element to psychiatric conditions like these, but without her input there are a great number of important but unknown possibilities that might influence her behavior. For example, does she use alcohol or recreational drugs? Is she on a combination of meds that might cause anxiety or panics, inattention, or cognitive effects? Has she had her thyroid checked lately? Is she in a relationship (could it be that be that she is being personally abused in some way, or has some sort of reality based fears of being stalked? I wouldn't assume that she has suddenly developed a serious mental illness, as seems to be the fear in the back of your mind. From what you have shared, there seem to be a lot of possibilities.
Does she live in the same residence with you? If so, it could be easier for you to approach her. If not, maybe you could invite yourself for a sisterly overnight slumber party so you can express concerns about the changes you are seeing and ask her to help you understand so you can support her.
Best wishes to you and your family.