I would establish two separate domains for each child’s toys and would not, at least for now, allow any invasion into the other’s domain. Your son may be old enough to have shelving for his toys, but your daughter probably needs a toy box. If the children have separate rooms, this will be easier than if they share a room. However, if you are clever enough you can find a way to keep things separate.
I also would use masking tape to label every toy. That way, if there is a dispute, you can simply read the name on the toy and announce, “This toy belongs to _____, so let’s not argue about it.” You will probably want to establish a third domain for toys that belong to both children.
Incidentally, this pattern tends to work both ways. In fact, it is more common to find that the younger sib constantly wants the older child’s toys. It is quite possible that your son’s problem started this way, and that he began to insist on controlling all the toys in order to keep his sister from acting as though she had a right to play with his. So part of your plan has to be to enforce the rule with your daughter: No playing with your brother’s toys without invitation or permission.
Chances are that after you establish and maintain this policy for a few months, you can relax the rules and allow them to play with one another’s toys. You will want to reinforce the policy frequently by remarks such as, “This is necessary until you learn to share.”
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.