Since the thread is still alive, sure. I have a sister who's 5 years older than me. (For the mental image builders out there, we're both blonde, blue-eyed, and she's tall.) When I was a little boy, she used to "play with" me when she was left to babysit me. Most often, she would jerk me off in the shower, though I was young enough at first that I couldn't really come yet. Then, at some point when I was 12 and starting to really get into the whole thing, she stopped, and told me it was gross now that I wasn't little anymore. Making me one of the few people to ever get dumped by a family member.
Incidentally, guess who I'm never going to let babysit any sons I have?