P.K.
You won't have to wait until he is 7, but an 18-month old can certainly play with toys by themselves. They just don't like to be away from other people. Putting him in his room is not an option at that age. A compromise would be to have a basket of some toys in a room that you are often in. Like the kitchen or family room. That way you can go about your work and he can play while he still sees you. Also, schedule some play-time with him so he gets mommy time as well. In the kitchen you may want to have a low drawer or cabinet for him to "get into" filled with tupperware or things he can pull out and bang on or play with while you fix lunch or dinner. He gets the social comfort of being around you or sister, and you can still get household work done.