What medication is she taking ? After my transplant,( in 2009) they had me on morphine and yes, I was indeed hallucinating and delusional. Morphine also can disrupt sleep.
There also is a syndrome called surgical psychosis which is common after something as traumatic as a liver transplant. My guess is the reaction may be a combination and just because your sister's reaction may be more extreme doesn't mean it isn't abnormal.