the ghost was of the mother and not the wife. The mother thought the main character was her husband (mc‘s father) so she thought he was cheating on her with the maid so she killed her (the wife) and her sister. And the last panel says it all.
no his mother was assulting him at night when he wents asleep and as shown his mother got incestly-pregnent and also she has some kind of undead like powers , I think she was a diffrent creature from normal animals like Vampire,ghoul,zombie,undead,humanoid-monster type , and yea when a strong female monster with high intellect looses his partner they loves and gets any trauma then they make incest relationship with their partners and their nearest offspring(son in 99% cases) and yea it doesn't matter cuz'it's natural or think what U want , now tell me really did u never watched or thought about incest-sex~ with family , and if not then Y r u here in this comment section and in this manga page~LOLOLOLOLOLOLOLOOOL~~~!