Same as what Donna and Sally said. We had mommas and kitties on the farm when I was growing up. Even when they grew up around them, the daughter and son kitties still treated the momma kitties as just "other" cats.

I had one cat when I was in my 20s, and my mom and dad's neighbor had the brother. They used to play together all the time as kittens. Once they hit about 9months to a year, forget it. They acted like they never even grew up together.

I had Taz and Finae for 5 years. They were brother and sister. Taz was with my mom and dad for the first year of his life, then when my Lucey developed a severe allergy and sinus infection to Bethlehem (she sneezed blood.. it was sooo bad, meds or no meds), so we swapped cats. Taz and Finae loved eachother, but they still behaved like cats that were just housemates. Not siblings.