For example if Root element is A and it has two child elements B & C. I have retrieve C by using some condition now with these How can I find Sibling B element? in csharp.
There are tons of ways, depends on what you're end goal is and the scale of the search.
The easiest way is to go to the parent element. The parent AFElement will have as a property a collection of AFElements: AFElement.Elements Property
You can enumerate these elements using a LINQ query for example and filter on name.
If you are doing searches for (or through) a large quantity of elements, use the AFElementSearch Class:
As a search filter you can specify the name and the root: AFSearchFilter Enumeration
Retrieving data ...