4yr old and playing alone

We have 2 kids, 4 and about to be 7. My son is the 7 and has always enjoyed playing independently. He plays well with his sister but after awhile he does prefer some space. My daughter has never enjoyed playing alone/being alone. Sending her to her room is the ultimate punishment. She won't be in a room by herself. She's very independent in every other aspect, far more then her brother ever was (insists on doing everything by herself, learned how to do stuff much earlier, etc) but hates physically being alone/playing alone. She can't play with her toys for very long next to her brother playing with his cars, he has to be playing WITH her. Or we have to play WITH her. I don't know how to cultivate this skill in her. Are there any books out there? Tips? via /r/Parenting http://ift.tt/2oigNDl