You have compassion for the mentally ill and that is genuine and heartfelt. The next question is how can you best harness that motivation to do the most good in the world.
MD/PhD is meant for people who will spend 80% of their time in the lab, doing research and, as time goes on, writing proposals, mentoring trainees, and lecturing. If that sounds like what you want to do, then you will need to find ways to get research experience while in school, during summer programs or even for a year or two after college graduation. The strongest candidates for MD/PhD are strong candidates for the PhD and have proven that they will be worth the investment of time and resources that a school will dedicate to you.
At the same time, an MD/PhD is expected to provide patient care in about 20% of time. This often means having clinic one or two half days per week and being "on service" one or two months per year during which time you care for hospitalized patients and supervise the trainees (students, residents, fellows) on your team. In preparation for a career caring for patients, you need to have some exposure to patients who are not your loved ones and the settings in which they receive clinical services. This can be paid or volunteer.
There are many other ways to improve the care of the mentally ill through clinical care (e.g. nursing, clinical psychology, clinical social work -- one of my close relatives took care of veterans with schizophrenia for more than a decade as a nurse in an inpatient unit and in a community support position), or in public policy through a degree in law, public health or public administration.
Take it one step at a time but know that depending on your strengths, there are many paths to your goal of making life better for the mentally ill and their loved ones. Kudos to you.