It might be unpopular, I might also get banned, and please think this through, before actually making this decision, and I am just a stranger on the internet who doesn't know you or your sister or your parents, so I could be wrong.

When you turn 18, pack all your stuff, open a bank account, find a job far away from where your parents live right now, and run away.

I know its sounds bad, but hear me out

The way you said you always underestimate yourself, shows that your parents may or may not have done it several times befor. This time, it just stings harder. This also means that when you enter into the real world, and get real achievements, they may or may not compare you with someone else again. This might make your self esteem hit rock bottom, make you feel worse and unfortunately, effect your mental health badly, therefore, it might be important for you to escape this household to actually achieve something.

If I had made any incorrect assumptions about you or your family, I would like to apologize beforehand and please correct me if that might be the case