Tubby & Peanut aren't related in any way except that they are both cats. But I still call them brother and sister as in "Tubby, where's your snotty sister?"

"Casper and Kitty's dad was black, just like Casper, and mommy was more or less the same as Kitty is now, a bit more greyish. The other sister looked like Kitty too, also a bit more grey."

Seems that daughters take after their mothers and sons take after their fathers. Interesting because that happens so often in humans too.