This could also be f/f daughter. Daughter finds bio mother and build an intimate relationship. It could also be bio daughter finds bio sister or brother.
I would like the adopted char to not let on at first who he or she really is. Perhaps he follows them around for a while and finds a predictable and convenient place to meet biological relative. Perhaps daughter find mother is a therapist and goes for therapy and hints at things which bring out the truth.
May have elements of dominant mother thinking she is showing her son how its done and he give her advanced lessons. Different character or it would also be fine without these elements.
WOULD LOVE OTHER SUGGESTIONS OR IDEAS OF YOUR OWN IF YOU HAVE A CRAVING AND HAVE YET FOUND A PARTNER FOR............
Thanks for reading.