My daughter is exactly this way. If we aren't home for nap time, she won't nap at all. She is 2 1./2 and my next one is due in the next few weeks. I will say her napping only in the bed at the beginning was tough, as i felt like a slave to her schedule, BUT it has turned out to be a GREAT thing. I konw that I have the same downtime every day. i can plan on things being routine, and so can she. Friends and family all know that she sleeps in bed, so they understand when we have to leave some place, or can't do something, due to the timing.
I have my stepson for the summer, and he is 8 and LOVES to be outside. So during nap time, I take him outside and he can play with his toys with no problem. he loves his sister, but he likes to be able to do his own thing without having to constantly help her.
One thing i would caution about though with constantly having to entertain your son, is that over time it will get worse. You should encourage him to use his imagination and play with toys on his own. I have a friend who ALWAYS entertains her kids, and never let them to do anything on their own, becasue they were the same way, would be bored and sassy. Well they are now all in school, and having issues with school. AND when we go over there to visit, we have to wait until the kids are in bed, which can't happen since my daughter goes to bed then too, otherwise, the kids won't leave us alone.
I know they have books out there that have TONS of ideas for activities. I would go and check them out at the library. or look at some homeschool books for preschool/kindergarten age, and see the activities they tell the teachers about. then you can plan different things to do while sister is sleeping.