Toys can produce problems for sure. Sometimes you find there are too many, and they keep coming. Siblings fight over them, you step on them in the dark and trip over them in the light of day.
The shared toy dilemma can be resolved by keeping the toys in one area that is off limits to the children. You maintain control of them by checking one toy in and out like the library checks out books. Clearly explain to both children that if they continue to allow their sibling to play with their checked out toy when either one of them loses their toy privilege, then that consequence will result in no toy privilege for the offending sibling, twice as long.
Do not allow your eight year old to break you down when you must exert your authority. Now it may take some organization to get started, but I guarantee that any chaos of toy sharing will be nicely resolved.