Do Siblings count as hooks in colleges like upenn or other ivies?
No.
They don’t count as hooks.
Some colleges do ask if you have a sibling or relative that attends- not as a “legacy” hook but because they might be curious to know why you’re interested in the school, etc. But having a sibling on its own cannot be a hook.
At Penn it might help if you are a twin, but in general, it is still not a hook.
http://www.thedp.com/index.php/article/2011/02/siblings_get_a_boost_in_penn_admissions