What does “this circumstance” refer to?

This event alludes to the poet’s current state, in which she is overcome by the agonising remembrance of her deceased mother. When the poet looks at the photo and recalls her mother’s laughter, she also realises that her mother’s age in the photo and the number of years since her death are the same.

Circumstance 

A circumstance is a state in which something is happening. Let’s say you’re at a business dinner and due to certain circumstance you’re accidentally given psychedelic mushrooms in your ravioli.

Circumstance comes from Latin and means a situation around something. If you are born in a conflict zone, you are born into difficult circumstances. Information about each candidate’s situation is needed when deciding whether to award scholarships to those most in need. If you missed your homework because the power went out overnight, tell your teacher that you were blind enough to read under the circumstances.