Hi,
Are you saying that some Sibley family members are on WikiTree, but your branch of the tree is not?
The first thing to do is become a WikiTree member. You'll need an invitation. You could request one from whomever added the other Sibley family members. Use the "private message" link you'll see on their profiles. See http://www.wikitree.com/wiki/Community_Membership for more info.
Chris