You had two bad things. You had the neglect. You had a sibling that got a better treatment. It is quite common that you get an extra set of issues because you are treated worse than a sibling. It really hurts a child.

Bad parents choose kids to treat well and kids to treat bad. I'd say this behavior stems from evil.

Building yourself up you have to love yourself like raising yourself emotionally, to be able to love yourself. It is circular yea, but you can do the actions even if you don't really feel the love behind them. But maybe something good will come out of it?