I believe it was Mary Renault who wrote that Ptolemy didn't think of himself as Philip's son so much as Alexander's brother (or something to that effect; I can't remember the precise wording). When contemplating Ptolemy's relation to Philip/Alexander, I like to keep that in mind for the reason that the word "brother" can have more than one meaning.

Ptolemy certainly had a brotherly relationship with Alexander, and if you take the "brotherhood" meaning of the word (Such as shield brotherhood) Ptolemy is also Alexander's "brother".

More to my point, if Ptolemy considered Alexander a brother and vice versa, I'm willing to accept that their relationship was that of brothers. In the same way that adoptive parents can still be "mom and dad", Alexander and Ptolemy were brothers.

Whether Alexander and Ptolemy's brothership was at least at first based on the belief that they were related by blood I can't say, but certainly that was not the only reason for their lasting trust and affection.

As for actual blood relation, I think it's as likely as not; if for no other reason, merely because I have no reason to believe that Philip didn't have bastards all over Macedon and maybe the rest of Greece.

~Beryl~