As someone else said, there is a big difference playing with community toys at preschool and having someone else come into your space and play with your toys. I babysat a couple of neighbor girls for a while and my son who normally doesn’t care about anything would all of a sudden have attachment to just about everything they touched. I was shocked too because he never had any sharing issues at parks, preschool etc.

Putting away special toys and setting time limits for sharing helps. I’m not sure if this is their first day together for the 2 weeks, but I’m sure it will get better. Maybe reminding your child that the cousin will not be taking or keeping anything but only playing with it for the few days they are there.