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 . ;)
However, at the most competitive colleges (e.g., the Ivies and their ilk) sibling status means next to nothing. There are simply too many siblings who are eager to enroll ... and too many non-siblings who are just as qualified (or more so). But, at the majority of other colleges, the sibling "hook" counts at least a little.
College officials do not typically comb through applications or transcripts to check out the matriculated sister or brother. They simply note the connection and may use it in the candidate's favor, if there are close calls to be made.