Sometimes family members can see the same therapist if they want too and if the therapist is willing to see the same family members.
Some therapists will not want to see family members from the same family because it can be a conflict of interest and make things not work out correctly when doing therapy for the same family members.
Most therapists will suggest that the other family members see different therapists.
That's because some therapists are not sure or confident that they can maintain their neutrality when it comes to providing therapy for members of the same family.
Actually it is better for the family members to see different therapists for the best results even if the therapists is willing to see the same family members.