Hi, it sounds like you are having a pretty rough time at the moment. Just from reading your post, you sound like you have got loads of great qualities-you don't seem to begrudge your sisters their talents and you obviously care so much about your family. Just a thought, but maybe you've got into a position where you think your rubbish compared to them and so you exaggerate their talents and only reflect on what you haven't got, instead of what you have. Why don't you take up something new, something that only you do, find your niche as such! It might take a few tries to find something you like, but try and join some societies at school, college or where ever. This may sound a bit twee, but try some volunteering, it honestly makes you feel good, plus you meet loads of new people (daunting but worth it!)
Basically, whatever you think of yourself, you are not stupid, and just because you feel you haven't realised your potential yet, doesn't mean you will never reach it!
At the end of the day comparing yourself to people constantly is not helpful in the long term, and although it's hard when it is shoved in your face at home, please try not to for your own sanity!
Btw-try planning your gap year in EXTREME detail-it will make it seem so much more real!
Hope you feel better soon :-)