Y'ALL
I went back to re-read this after the latest chapter and know that we know what grandma looks like, this page https://dynasty-scans.com/chapters/vampeerz_ch14 make so much sense.
This means that her grandma was ALSO one of those few people capable of wielding the sword. We had a little bit of foreshadowing of this on this page too: https://dynasty-scans.com/chapters/vampeerz_ch03#19
when Ichika refused to kill Aria and she said that reminded her of Chiyo, before being cut off my Ichika.
Someone earlier said that from the looks of it, grandma may have tried to kill herself so maybe because she had tried it and knew what it felt like, she refused to take Aria's life?
Sorry if it's kind of a lackluster analysis but no one had pointed it out and I just had to.