Ptolemy gives brothers and sisters a chapter of their own in the third book, and he does not read them from a third house. He reads them from the parents, on the plain ground that siblings are the other children of the same parents, and takes the maternal place as his starting point.
This is a different kind of astrology from the house catalogue, and worth seeing clearly. The Hellenistic method asks what a thing is derived from and works outward. The medieval method assigns each subject a fixed compartment and looks in it. Neither is a corruption of the other. They are two solutions to the problem of getting from a sky to a life, and the second is far easier to teach, which is a large part of why it prevailed.
The two can also be run together, and traditional practice often does. A reader may take the third house for the sibling relationship as the native experiences it and Ptolemy's derivation for the family as a fact of birth, then treat a disagreement between them as information rather than as a mistake.
What neither method delivers is a count that can be relied on. Both describe ways of estimating the number of siblings from the planets in the relevant place, and both hedge the estimate with so many conditions that it stops behaving like a number. Astrologers who have promised an exact count from a chart have been embarrassed in every century, and the honest reading of the tradition is that counting is among the things it does worst.