@tsmox She also seems to be in a single-parent household so it's unlikely that she's adopted, but she also corrected herself to refer to Shimako as "mom" when speaking with Shizuka so it's unlikely for her to still be in contact with her biological mother. My headcanon is that Shimako is her aunt and her parents died when she was old enough to recognize that Shimako is not really her mom but young enough that she views Shimako more as her mom than she does her biological mother
I don't know why I put so much thought into the life situation of a character that's only present in 2 chapters of a 14 year old manga...