Midlife Crisis Don't remember if married or just live together with their daughter. Both sister and niece can be LIs to give a time frame of how long to sister has been with the "husband" Some places would count it as common law marriage.
Options on first visit...
With MC's wife: 3-some (MFF) or wife swap
Without MC's wife:3-some (MMF) or cucking husband (with optional t-bagging)