What does "Mother's husband's sister" imply in terms of family relations?

The phrase "Mother's husband's sister" refers specifically to a familial relationship within the context of a family tree. To break it down: "Mother's husband" is your father, and therefore "Mother's husband's sister" is the sister of your father. In this context, this person is referred to as your aunt.

This understanding is important in recognizing how familial terminologies work and their relationships. An aunt is a parent's sister or the sister-in-law of one's parent, which aligns perfectly with the description provided.