How to specify "1 dog, 2 only if from the same family" on my calendar?

I am only willing to do two dogs per night if the dogs are from the same family (I live in an apartment and have my settings to only small dogs, but I know that small dogs often come in pairs. Two small dogs for the weekend could be a bit much the first few times, but if they are "brother and sister" they will be great together). I don't want my calendar to ever show 1 and then someone else ask for the remaining.