Sanae has a frog theme much like Suwako. Cirno freezes frogs for fun, so they are natural enemies. You'd think the hostility would be from Sanae toward Cirno, but I guess that's the humor.
Are you looking for the Scarlet Devil Mansion?I hate you! I super hate you!"Natural Enemy"*Flick*Ah.Cirno-chan.SDM...SDM...Oh, Sana-I bet you don't know where it is, right? Just put your mind at ease! I'll tell you!