Wow, that's a tough situation. My usual style is to blame the baby, lol, but she doesn't seem to get it as you've hinted at naptime. It's so hard when they're family AND a bit er, sensitive.

I know you don't want to be direct with her, but maybe there's a way to say it sweetly enough that she will accept it. eg- "I so love your little visits, it's a shame we don't have more time to chat together since it's always naptime when you arrive. How about we arrange a time every week to catch up? (insert suitable time and day here) Great! I can't wait to see you next week, I have so much to tell you."

that sounded better in my head. lol. But sometimes people respond in a more positive way if you make it about how fabulous they are, instead of how inconvenient the time they choose to 'pop in' is.

My only other idea is to lock the door and hide. Obviously, that's not exactly a long term solution.