It's little use to read a thing three times if your eye skips over the operative kanji each time, confident in its first heat-induced misunderstanding. Mrs. Watson is the landlord's mother, not his daughter. When startled she looks like what she is uhh was?--an old woman. The zombies crawling through the window are thus her friends from the graveyard in back where she's buried and 'Gramma' is, as it often is in Japan, a pet name. End mystery.
I still wouldn't recommend the thing, which out-Guignols Guignol. A fondness for drawing innards is a mangaka specialty.