I know the title says Adult Children. But having read it, I think so many parents would avoid these Adult Child issues if they set some boundaries with their children when they were young. Start by just saying NO occasionally. Children need to hear NO and parents need more practice saying it. Pick times when it's a no-brainer to say NO. Then stick with it. It gets easier.
This is the Boundaries book I recommended earlier.
This is the Boundaries for kids book by the same authors. They are great authors.
Drink some water. It keeps your brain working right.