When the cousins come I would allow them to play with whatever is available and out to play with. If there is something special, delicate, breakable, put it away in advance.
When the chldren are finished playing tell them it is time to 'clean up' and that all toys go in their respective places so that they will be available for play the next time the cousins come over.
If you get a fight from the cousins gently remove (PRY) the toy from their hands and say, "No Louis, this toy belongs to Johnny and we will have it here for you to play with next time. It lives here and here it has to stay."
Your kids wil love you for being an advocate for them.
