This poem or song,while a tad more complicated than was the case of Emily May (PETERS)MCDONALD, relates a similar situation. Emily May was only 4 years old when her mother died and Jane Elizabeth MCDONALD cam to care for the children and kept house for the family. When Jane Elizabeth married Emily's father,Jan's younger brother, Herbert Lloyd (Bert) MCDONALD became Emil's step-uncle.
Bert was only 6 years older then Emily, and when Emily was in her teens fell in love.
Father George did not approve of the match, so Emily and Bert eloped, and went to live in South Australia, where they stayed for a number of years.
This of course brought up the interesting situation mentioned earlier.
As husband of her husband's sister, her father was also her brother-in-law. And as sister and brother-in-law to their father, Jane and George were the children's aunt and uncle, but as father and step-mother to their mother, Jane and George were their gandparents.

Whew what a complicated mess.

A further complication occurred when Emily may's aunt, her mother's younger sister, Madeline (Madge)TRELOAR, married another McDonald siblings, Alexander (Alec),older brother of her husband, and younger brother of her step mother. So her aunt was also her sister_in_law.

I would love to find more information very interesting family