Of course, I’m delighted you want to be a psychologist. I can’t think of anything more exciting (except maybe being the next American Idol!). But seriously, you have a worthwhile goal and you have the academic ability to match. It’s sad you don’t have more family support but I do have a few guesses why your mom and sister can’t do that for you.
Have you ever considered that maybe they are feeling threatened by your goal? Many people have the idea that psychologists spend all their time analyzing people. If they are nervous about what might be discovered, they do all kinds of things to avoid it, including putting down the entire profession and anyone who is doing it. Of course, we psychologists don’t have our psychologist hat on all the time. It would be inappropriate, not to say exhausting. Most of us just want to enjoy our family and friends like everybody else does. We no more want to be over-thinking our relationships all the time than a doctor wants to be talking about someone’s gall bladder at a dinner party. Professionals save their professional advice for the office.
You didn’t mention how your mom did in school or how your sister is doing now. If you are a better student, it’s possible that they feel a little inferior to you. Maybe they think that putting you down is a way to keep you from thinking you are better than they are. In a strange way, that may be an awkward attempt to help keep you in the family.
Is it possible that you are acting superior? Even a little? Maybe your mom thinks she has to back up your sister because, purposefully or not, you are making your sister feel bad. Are you finding ways to let your sister know that her goals are worthwhile too?
Can your family afford University? Maybe your mom can’t handle it financially so she is trying to discourage you rather than disappoint you. If that’s the case, show her you that you don’t expect her to pay for some or all of it. Actively look for scholarships and loan programs and get a job and a savings account.
Without knowing you, all I can do is offer those alternatives. Try thinking like a psychologist to figure out what might be behind the way they are treating you. Once you have an idea, find a way to be in sympathy and to change your own behavior so your future goals aren’t such an issue. Keep family conversations to other topics. Quietly work toward what you want to do. You need your family’s love but you don’t necessarily need their support for this particular goal. Look for your support from your guidance counselor, teachers, and friends. Once you help your mom and sister understand that you aren’t making your choice to separate from them or feel better than them, they may come around.
I wish you well.