>>21291770
If Sumireko is Renko's mother/grandmother, for Renko to keep the Usami name then either Sumireko marries a guy with the same family name, her husband takes her surname or Sumi is a single mother. It's also possible they might be much more distant if pic related is actually true, and Sumi might be dead before Renko is even born. Sumireko could also just be one of those weird aunts in the family. There definitely is a relation though, as Renko produces a photo of the netherworld in her first apperance while mentioning some ambiguous stuff about connections while 16.5 is about Sumireko screwing around with her phone.