I liked this manga until this last arc.. I dont like how the sister faced zero consequences for wasting her brother's time to the point he missed his own child's birth, and y'know the whole conspiring to murder his wife.
Entire arc resolution can be summed up with a single Jeremy Clarkson gif.
So she either is responsible or he blames her for his death in some way, and looking how she is keeping him around despite the attempted murder, I'd say she does too.
"could you tell me the appeal of muscle training?"
"yes, I could squeeze your head until your eyes pop out like you're a 90s cartoon character in love."