A child doesn't have to share his possessions. Warn him, however, that sometime he may want to play with his sister's toy and expect her to share it with him. She may not feel like sharing it if he has not shared his toy with her. Teach your children to take turns playing with family toys such as video games or board games. Also teach your child to share toys when friends come over.
Sharing is a necessary skill for making and keeping friends and getting along in school.
Younger siblings often intrude on older siblings' friendships and play. It is helpful if you give the younger sibling a playmate or special activity when your older child has a friend over. Protect your child's study time from interruption. Designating a study room often helps. Avoid showing favoritism.