Rahu and Ketu are made use of so correctly for predictions both according to their presence in a horoscope and according to their dashas in the vimshottari dasha scheme. They are not planets, yet they are treated as ones, and their effects are shown through the proper study of the horoscope. The ancients must have combined their actual observation with their yogic insight, since India has always been a land of the greatest yogis in the world, to arrive at the clearest discernible results. 

A very simple rule may be tested. The 3rd house is the house of younger siblings and the 11th house of elder siblings. Rahu in those houses alone makes one either the youngest or the eldest of the children of the parents or the eldest or the youngest in that sex. 

Rahu who is the cause of an eclipse here does not allow the others among the siblings to occupy the first or the last position. Rahu, the cause of eclipses, performs the same act of eclipsing the siblings here.

reprinted with permission