Before big events, I often find it helps my son to get a "preview" of what's going to happen (and what's expected of him) by writing a story about the event.

1. Our block party!
Today is the Block Party! We look forward to this party all year. It's so fun!
At about 4pm, we will put flags up at both ends of the block so no cars can
pass through. Once the street is blocked, kids can safely ride bikes and
play in the street. There will be lots of kids playing outside. You and your
sister will be, too! The kids in the neighborhood are your friends. They like
to play with you. They want to have fun, just like you.
At about 5pm, the grownups will come out with food, tables, chairs, and
BBQs. The grownups are also excited to talk to each other and be friendly.
At about 6pm, dinner will be ready. We will call you to eat at the table.
There will be burgers, corn, and lots of other salads, snacks, and desserts.
Daddy and Mommy will be busy setting up, but we are always there to help
you if you need it. We want you to have fun and play with your friends and
enjoy the party.
To have fun with your friends, here are some things to remember:
* smile and say hi
* share your toys

2. * include your sister in your games sometimes
* suggest something fun to do, or ask to join a game already happening.
* use a friendly voice (no yelling)
* use friendly words (no name calling)
If you have a problem, Mommy and Daddy will help you fix it.