The page where they arrive at his door and meet Nico provides a lot of information.
Hiyoko doesn't react in any way, hinting that she's used to seeing Nico and most likely has done her own research on ghost phenomena, including details like them only being visible to people who are intimately related to them.
Nico herself not remembering them until this chapter might be a hint that her time is running out and she's starting to fade away. It's important to remember that ghosts are restless entities bound to the mortal realm due to some factor that prevents them from moving on, such as dissatisfaction or grief over unsettled matters in their lives. Nico seems to be feeling more and more content seeing Tsugumi grow with every chapter, which is the matter she wanted to settle, until she's finally ready to go.