Because Nina wasn't at the hospital and Jonah was. RZ would have been communicating with him if there were any developments. She had already spoken for quite a while with Nina earlier in the day when she picked her up at the airport. RZ was engaged in a conversation with her sister, giving her updates.
Most likely, once she finished texting with her sister, she went to take a shower and get clothes ready for herself and JS for the next day. Based on evidence & blood patterns, Keith Greer thinks RZ took her shower. Put on the towel and left the bathroom to go into the master bedroom. Something caused her to go to the bedroom door and, based on blood patterns on the carpeting, she stood there for some time talking to someone (probably Adam, he was the only other person on the property). At that point, RZ was forced down the hallway to the back bedroom. She struggled again at the door to that room where her towel was pulled off. More blood stains were found on the carpeting there in the hallway where the towel dropped. Farther into the balcony bedroom, more blood was found on the carpeting, indicated she was laying there, nude for a while, probably when she was tied up.
Greer estimates she could have been killed by 11:30 pm
Source: Tricia's podcast with Keith Greer just before the wrongful death trial began