From Peanuts Wiki:

Belle is Snoopy's sister. She first appeared in the comic on June 28, 1976. Belle only made a few appearances in the Peanuts comic strip. However, she is well remembered because of her use in Peanuts related merchandise, including Belle stuffed toy animals which were produced in the 1970s and 1980s. Due to her few appearances many people who were not regular readers of the comic strip mistakenly thought that she was Snoopy's girlfriend, rather than his sister.