Family therapy varies almost as much as the families themselves do. Sometimes family therapy really is an entire family. Often, it involves a mix of subgroups within the family. These options are up for discussion based on client preferences as well as presenting concerns and best practices given the issues families are seeking help with.
While some therapists prefer not to work with individuals within the same family, in many cases, I prefer to do so if multiple family members will be seeking therapy. My point of view is that individuals working with the same therapist concurrently encourages a smoother change process as the fuller context of each individual and the family can more easily be considered in treatment planning. This approach also lessens the likelihood of error or mixed messages to clients based on multiple therapists having difficulty coordinating with each other as often as would be ideal.