What to do about a younger adopted sibling that always divides the other siblings and causes money problems for my parents?

I would say let your Parents do what they desire to do.It is their money.If the Parents want to provide or to help their children with money problems so be it.
Sounds like it's not the parents involved it's the siblings not getting along or not seeing eye to eye on things.If one sibling is doing well for them self and the other needs a little help thru life.The sibling should help the other sibling.Adopted or blood doesn't matter.Family is family and should stick together no matter what.
I've seen families break apart all the time over greed.Don't let the money get in the way of your sight on things.Nothing wrong with helping one another.One day the tables may turn,keep that thought into mind.My Parents had four boys.Our parents devorced during the 1970's era.Our Mother raised all four of us boys by her self.Our Mother told us boys when we was children this,No matter what!,You four boys stick together!...Our Mother passed away.Today us boys are in our 40's & 50's and still take care of one another when needed.Why?Because we are all we have other then our own wifes & children.
It's easy to hate and point fingers,it takes a weak person to do that.It takes strength to be kind to one another.It's nice to seat back with your siblings to talk & joke about the past.Being apart from your siblings makes a lonely life for your self when it's preventable.Greed is the cause of family relationships to go bad.

I agree with blannie, we don't have enough information to be able to help.

ferris1, I sort of agree yes the adopted sibling, if it is a full legal adoption, needs to be treated same as the biological siblings.....however from the little tiny bit of information given, it sounds like there is some dissension amongst the siblings.
I wonder if there is a huge age difference, or other mitigating factors, which would cause resentment or dissension?
Again we need more info.
Also, have the parents made a will, and included all the children equally? Are there reasons the parents might have excluded some of the kids? It happens.

First, an "adopted" sibling should be no different than your biological siblings. Your parents chose this person for better or worse, and instead of looking for division in your family, all sit down and discuss how best to help your parents as a family. If your parents want to give this sibling money, they may do so.

If it is that no-one else cares, are you alone in this? If you're alone in this, maybe there is not much you can do. If you're going to take on some role, maybe it would be financial POA, to guard against the younger sibling, but only if your parents need/want this, and depending what their current state is.

Sarahjane how old are your parents? What (if any) medical issues do they have? They need a caregiver, it sounds like? Tell us more so we can try to help. You haven't given us enough information about your situation to be able to comment.