Knowing me - Evelyn Hood

Rachel Carswell is in her mid-thirties at her father's funeral when she learns that she was adopted at birth by the couple she has always looked on as her real parents.
Much against the wishes of her accountant husband Martin and her adoptive mother, Doreen, she starts to search for her roots, and meets twin sister Cara, who turns out to be the woman Rachel could well have been, given a different upbringing.
While Rachel's staid husband and her adoptive mother, Doreen are still trying to cope with the sudden intrusion of Cara and her easy-going, attractive husband Angus into their lives, Rachel and Cara search for their biological mother.
And again, Rachel's formerly serene life is shaken to the core.