Also I found out that my sister like many owners, by mistake has made the bird think she is her mate......
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Thanks for helping this bird (I didn't say that in my first post, but I should have). The absence of your sister and the addition of a new home could definitely cause anxiety. That having been said, since the screaming pre-dates the transition, it could be a mix of behavioral issues, anxiety and hormones. All cockatoos scream, but what you are describing sounds excessive. The good news is, that means you can fix it (but it will take time). In conjunction with the advice I posted above (regarding behavior) you should also address the potential hormonal issues that could be going on here.
Hormonal birds tend to make more noise. To help with this, make sure she is getting a solid 10-14 hours of dark, uninterrupted sleep each night (12 is about the bare-minimum for mine) and then try to get her light schedule regulated. How much sun-light does she get during the day? Too much sun can stimulate hormones, but too little can be problematic as well. A solid bedtime/wake-up routine can work wonders.
Make sure you do not allow your bird to play with boxes or hang out in shadowy places (such as, under furniture, in piles of blankets, in pillows, under clothing, in crumbled paper or shredded materials etc). Cockatoos are easily stimulated and so you want to make sure you remove any triggers for nesting behaviors. Shadowy areas are a major culprit.
Also (and very important) no one should pet the bird anywhere other than her head/neck. If you have touch touch other places for medical reasons etc, that is different, but stroking under the wing, down the back etc is VERY sexual for a bird and it sends the wrong message. The same goes for hugging them against your chest or even allowing them to sit on your lap under a blanket. It is like leading a bird on and it can be very upsetting when you do not mate with them (like rejection). SO, please tell this to your sister as well.
You may also want to consider limiting or removing access to
certain types of shredding toys. Chewing toys and toys in general are great for a hormonal bird because they need to stay busy, but the sea-grass-type of shredders (soft grass toys) can make thing worse for an already hormonal bird (it just depends on the bird). Wooden chew toys etc are great all of the time though. If you get new toys for her, make sure that you introduce them slowly (put them within her line of sight for a few days, but outside of her cage so that she gets used to them). New toys are often a source of fear for birds.
If your bird ever regurgitates or engages in other sexual behavior towards you or your sister, change the subject immediately- withdraw attention or redirect the bird's attention to something like a toy to change the mood/tone of the situation. Do not make a giant deal out of it (never scold), but do not attend to it either and stop doing anything on your part that might be triggering it (if the bird starts quivering when you are petting it---even if your are only petting on the head, stop).
Warm/mushy foods can make hormonal issues worse because they are similar to regurgitation, so keep that in mind when feeding her. If she isn't already, make sure that she is eating pellets and a bit of healthy seed mix (no peanuts, corn or sunflower seeds). Diet can have a huge impact on behavior.
Random thoughts--- If you are leaving the house and want to know if she is screaming/what she does when you are gone, you can do what I used to do by live-streaming her to an "unlisted" Youtube Live-stream. Just set up a Youtube account and login on a laptop. Use the laptop camera or a web-cam to record the bird and before you leave, email yourself the link--then start the stream. Anyone with the link can view an unlisted stream, but other people will not be able to (as long as it isn't on a public playlist). You can then use your phone or another computer to check in on her in real-time (and this way, you will know how many cookies and apology notes you need to bring your downstairs neighbors lol). When you get home, hit end stream, and once the video processes, you will be able to review her entire day (assuming your internet connection didn't time-out). During this time, you might try experimenting---see if leaving music on makes your departure from the house better or worse....try different genres of music (birds DO have very individualized preferences when it comes to music). You could even try other sound-tracks (you can find 10 hour recordings of people talking/crowd sounds, babbling brooks etc)...
You might also want to consider her cage location. Is it currently in the part of your home where you spend most of your time? If not, it should be. Is the cage exposed on all sides, because sometimes parrots feel vulnerable to attack (instinctual fear) and it can help to have at least one of the cage sides backed up close to the wall (this varies among birds, but in general, it is best not to have a cage in the middle of a room). In some birds, windows can also be a source of anxiety, but it is a double-edged sword, as natural light is extremely important for them as well. That having been said, too much natural light can cause breeding behavior. Whatever you do, make sure that you are leaving plenty of lights on for her during the day (whether or not she is near a window) and definitely make sure she gets some natural light (if not when you are gone, when you get home).
If you think lack of sleep is an issue or that she is staying up at night because of you, consider investing in a smaller sleep cage where she can spend the night in a different and quieter room.
Do you cover her cage at night and does the cover fully block out the light?
I have a great cover that I purchased online and it is breathable but it also is also very effective at blocking light and it covers the entire cage (
http://www.cozzzybird.com/ -I got mine on Amazon and I LOVE IT).
Does she get plenty of exercise and time out of her cage, because if not, that could be a major issue as well? Cockatoos need quite a few hours of out-of-cage time each day, and at least a solid hour of play/interaction. Passive interaction is fine, but it is no substitute for direct, one-on-one game playing etc (assuming your bird likes you enough to do this). If she is still fearful of you, then don't push playing etc until you have a better bond, but do allow her to spend plenty of time out of her cage (Assuming it can be done safely).
Finally, if she is generally sweet and charming when people are around, you might consider introducing her to your neighbors. They might be more understanding if they felt a connection to your bird.
