Mrs Hudson was physically abused by Mr Hudson.
She didn't do anything about it until he threatened to
do the same to her 6-year-old niece. Mrs Hudson
decided Mr Hudson needed to go, so she framed him
for a "murder“ that was actually an accidental death.
Sherlock was called in to investigate and figured out
almost immediately that there was no murder. When
he saw the bruises on Mrs Hudson's arms, and the
way she and her niece flinched around Mr Hudson,
he made certain he was convicted.