See clip from Attorney Rajiv S. Khanna's conference call video that addresses this question.
There really is no statute law that specifically prohibits such an entry. Well maybe there is something that says you must have nonimmigrant intent but the question is can they do it. Theoretically yes they can enter on a b visa while the green card is pending especially when something is pending for 13-14 years they maybe allowed an entry but they can be disallowed an entry at any point of time. So there is no guarantee that the B-1/B-2 option either the renewal of the visa or entry at the airport will be permitted on a indefinite basis. You could be stopped any time. However H visa, L visa, E visa and O visa are some of the visas that are not subject to the problem of green card pendency. These visas can be utilized. Also remember a green card can be filed through several different categories at the same time. So if your sibling qualifies for other categories they can apply under all the categories available for them. So H-1 visa is no problem b but B-1/B-2 no guarantee.