my sister act like a 4 year old

I have a little sister that if she doesnt  get her way still crys and thorws a tantrum, and she is still talking a little baby talk every now and then, and she hate doing her home work,, my dad adoted them when the y was babys, my little brother 11 and still have actadents wet the bed, and of course my little she is still act like a 5year old what can i help do my dad is 73 and i have help raise them,