My wife and I decided to have a picnic with my sister and brother-in-law.  We picked out a park to go to because it had a pavilion we could sit under to enjoy the weather.  My wife and I planned to make a salad that had avocado and peppers in it.  My sister said that she would bring pizza and a dessert.  We prepared the salad by cutting up the lettuce and other veggies for the salad.  We put it in a bowl and took it with us as we drove to the park.  We found the place to eat and waited for my sister and brother-in-law to arrive.  After about ten minutes, they showed up with the food and the tablecloth.  We laid out the cloth and put the food on it and the plates.  We ate for the next couple of hours together there.
How many people went on the picnic?