What would be a polite way to bring this up/ do you think it's rude?

Its your sister for gosh sakes. No need to sugar coat things, just call her up and say "hey, I plan on bringing my dog this weekend beings you brought yours the last X many times. Just figured I would let you know so you can arrange boarding" I personally, prefer blunt and to the point. She'll get the drift and leads less to manipulation by being direct and ertive.