Really a him problem. My brother married my t's sister (like 40 years ago; none of these relationships exist anymore) so i am familiar with dual relationships. My t prioritised our relationship as it was the pre-existing one and she was the relevant t (brother and SIL also were t's). Also she told me she talked it over with her partner (agency and personal).

Another time, i had a severe rupture with another t regarding boundaries. Actually its what brought me to this website. I talked it over with a former t (my immediately previous t), a gf who is a t, and a
neighbor who was a doctor.

All that to say, get a second opinion. You can tell your t beforehand or not. But you are not without resources.