Sometimes young kittens forget where the box is, or wait too late to start for it. By restricting your space during these times, your humans will increase the chance that you will use the litter box. Everytime you use the litterbox, it will reinforce your using it the next time. Also, you will have your brother there to set a good example - and kittens do learn by watching. Don't let the humans think that your problems are because you are highstrung.
Most Siamese cats are very clean.
Don't let them label you. Your problems are because you are young, had a bad start in life, and perhaps are not totally healthy. I believe that if your humans are patient and loving, and if they try the simple methods I suggest, you will turn into a wonderful pet. Don't be afraid to write me again if necessary.