Your brother needs an effective advocate for his care needs while he's recuperating away from home. Your sister is nearby and actually works in the hospital across the street, that's great - she should raise your brother's condition with his primary care doctor, attending physicians giving care right now or who operated on your brother, the nursing staff at the rehab facility, and focus their attention on it. Many hospitals have drug interaction clinics known by various names, medical consult is one used at the UW Medical Center, they specialize in reviewing patients pre- and post-op for bad drug interactions. Find out if this hospital has one. Ask them (or rather ask the attending physician to prescribe) this type of service to visit your brother. Polite but firm usually does it. Medical consult exists primarily to foster good medical outcomes, and avoiding bad ones, which cost hospitals time and money through lawsuits and complaints.

As to signing a document, I can't speak to that - but it may be important to note that while in a care setting, if your brother is the only person able to sign documents and assist in making his care decisions, the nurse may have pursued the only avenue open to her in seeking his approval for his care, even if he was having a drug interaction. The fact that the nurse kept asking the same questions may be indicative that she was probing a difficult to assess patient for some essential level of coherent decision making, one that indicated he was cognizant of his health status and could make decisions for himself. I don't know your brother's status, if he entered care with a wife or someone else who he designated to make medical decisions for him should be become incapacitated. I can tell you that almost any medical professional would rather seek more care for a patient having difficulty with a medication, even transfer a non-responsive patient back to medical care than continue with them on their post-op service.

As MDinana says though, post-op/rehab facilities can be hit or miss, and pain management is the area that facilities tend to bull through and err on the side of management, on the understanding that a patient with normal vitals and restricted mobility may not tolerate the pain of knee replacement, but the nursing staff can tolerate confusion, twitching and fussing. If the pain management is short term that is. There are fewer physicians in a rehab facility to monitor their reaction to meds and other things. Its often left to what was prescribed on an outpatient basis, and nursing staff. Like I say, I've seen folks try to bull through pain management, its not always good, you need some good caregivers and usually some effective patient advocates with eyeballs on the situation pitching for other alternatives.