Although it is impossible to determine an exact waiting time, waiting periods for a sibling are very long. For siblings in Africa, you should expect to wait between 6-10 years from the time your green card petition is received by USCIS. For more information about the sibling green card process, please see http://www.whittfirm.com/family-immigration/family-green-card-sibling/.
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Right now, they are processing sibling cases that were filed 11 years ago. Sometimes the waiting period changes, but that is what the wait has been recently.
If it is possible, it may be faster for you to petition for your parent and then have your parent petition for their daughter when they arrive..
You may call me for a free consultation at (206) 724-1940.
Andre Olivie, Esq.