Your family might be reacting the way they are because you look different then when they last saw you. If they aren't seeing you on a daily or regular basis, then any change will look dramatic.
I will say though that you can't trust your own judgement. I had lost a lot of weight (being 5'8.5", I lost 60 lbs in about 8 months), and even at 152lbs when I looked in the mirror, I saw fat, yet my friends saw emaciated :wacko: (full disclosure, this was over 10 years ago and a gained a few of those back ).
If you have a trusted friend at school, ask them to objectively let you know when you look 'no longer fat'. They'll probably have a more objective opinion than yourself or your family.
But as you say, it all depends on your body type. 140 could still be fat for your body type, yet it could also be too thin for someone elses.
Another thing to keep in mind is that some parts of your body might not lose weight like you'd like. For example, say you can pinch an inch on your side/gut. You may feel that you can lose more weight, but if you try, you might just lose more muscle elsewhere (upper body, brain, etc).
So instead of exclusively using weight as your goal, make sure to keep in mind the rest of your body.
Also, you didn't mention how you were losing weight. If you are just watching what you eat, you may want to throw in some more physical activity. If you haven't already done so, you can start doing some strength training for your upper body (hopefully you have one of those free gym thingies at school). Focusing on your chest, back and arms will help diminish your midsection. As well you probably know that muscles weight more than fat.
Being at the gym also gives you the chance to look at some hotties .
Anyhow, good luck with your goals, it sounds like you've made awesome progress as it is. You'll be able to wash clothes on your abs any day now .
Take Care,
Vic