Evaluating conversational therapist competence in Motivational Interviewing (MI) requires multi-session assessment, yet current benchmarks rely on simulated patients that fail to model progression from session to session. We present MuSe-MI (Multiple Sessions Motivational Interviewing), a simulated patient framework grounded in Motivational Enhancement Therapy (MET), a multi-session clinical protocol of MI. MuSe-MI regulates dialogue generation through four evolving internal states from the MET clinical manual (anger, readiness to change, problem recognition, and self-efficacy), paired with an inter-session life event simulator and structured memory. Blind evaluations by licensed clinical psychologists confirm that patient dialogue reflects these internal states with high inter-rater agreement. Experiments show that MuSe-MI reliably distinguishes conversational therapist MI adherence levels based on patient language progression, establishing an automated multi-session evaluation benchmark.