Hi, I read your post, and i'm sure you are not a disappointment. It's happened to me before, but with confidence, and time, it got better.
I'm 27 now, and when I was 16/17/18, I was somewhat of a rebel. I was the black sheep of the family. Ended up getting thrown out and into hostels. My sister got A*, whereas I spent too much time skipping classes and walked away with C/D's. (Not bad, but not as good as 'my sister'). She was the pure, model child liking men, I was the strange one with dark hair, and piercings who liked women. Told I was such a disappointment and basically my family washed their hands of me.
I cannot describe how low I felt during those years, but I guess its similar to how your feeling now. Well, things will get better. I promise you that! I had to move away to a strange place, and sit in a self contained room, with no friends, transport, miles away from my family.
In time things got better. I stopped trusting people, and stopped giving a #*/$ what people thought of me. I passed my driving test, scraped money together and moved back to my home town. I got my head down in the hostel and resat GCSE's. I got a full time job on the back of those results, and I've been there 9 years now. I've also adopted a little boy who my mother loves to bits.
I didn't speak to my family until I was 21, as I was upset. I would sit and cry, and wish I had the courage to pick up the phone.
Fast forward 6 years, mums been very poorly, and I haven't been over well either. Both of us are very grateful at the chance of life again. I said to her when I started my degree 'I will graduate like my sister, I will make you happy'.
She replied with - you will always be brilliant to me. You don't have to do a degree to make me proud of you. I'm happy with whatever you do. As for my sister, she hasn't spoken to my parents in 5 years, (since she requested 'financial assistance for a mortgage', and was refused). They weren't good enough then.
What I am trying to say is, have confidence. Go downstairs, and have a convo with her (but don't get upset), offer her a cup of tea, ask if you can run her a bath. (simple thing I know, but it's the long term plan) When your off to bed, say I love you , it will prompt her.
At some point, she will realise you are a grown individual, and she still loves you. She wouldn't want to hurt you, or see your upset. Maybe if your of legal age, take her down the pub for a drink!
You only get one life, and one chance, so go give her a hug and a cuppa, and let her know, that even if she thinks your a failiure (which your not), then you love her. Maybe drop in a comment about 'aiming for a first to make you proud'. She might say that she is already proud.
Don't worry about friends for now, you will meet loads at uni.
If she doesn't soften, dont let it stand in your way with studies. Live your life, dont cry, and when your making a sucsess of yourself, she will realise how much you have strived.
Once your aunt's daughter is at uni, and then settling down and having children etc, then these friends will disappear. I only have 2 friends left from before I moved away. Friends you know at school/late teens, all grow up/move away, have kids. Don't base your life on how many friends someone else has!
Apologies for the long rambling reply. I've been on the merlot.
. Basically what I'm trying to say is don't give a frig what people say or think. Just be you. And if they don't like it, their loss.
10 years ago I was classed as a 'screwed up dyke of a sister'. Now I'm a 'mum', working, and studying, and closer to my parents than I have ever been. Sibling rivalry is epic, so maybe she was just trying to avoid an argument with her sister. As a parent, take it from me, I'm sure your mum would be feeling awful if she knew you overheard them.
Keep smiling, and good luck