My mother has recently devorced my father. Myself and my siblings feel nither parent is capable or sane, and should not get custody. I am oldest of the kids, my sister 19, my brother 16, also have a 12 year-old and 8 year-old. Is there a way for an adult daughter to get custody of my other siblings?

Very unlikely, the court would have to rule that both fahter and mother is unfit. Not just a bad parent but unfit. While the court will allow each child to speak ( maybe not the 8 year old) it is always up to the judge.
The oldest would have to hire an attorney and file for cusotdy,

If the mother and father agree and will sign them over, if not expect a hard court battle.