Does having a sibling who attends a college increase your chances of getting in? Do admissions officers go back and look at the sibling's application or look at his or her current GPA?
Having a sibling who attends your target college (or who is an alumnus) will usually work in your favor. Most college administrators feel that, by enrolling more than one member of the same family, it increases the odds that this family will feel connected to this college and thus donate more money ... and maybe cheer more loudly and enthusiastically at sporting events, too . ;)