Yes- I don't recall cross generation marriages or relationships being tradition in other societies.
In the Egyptian it certainly wasn't the case and the Achaemenian neither- Darius marries his sister- Statira- if I am not mistaken- so there are similarities between the Achaemenian and the Egyptian- and more recently and British royal family- preserving the blue blood I guess.
Greeks practised polygamy-and it may have been Philip's downfall this one..

Having said all that- when discussing such matters I am tempted to look for a different term than "married"- I don't think that their concept of "marriage" was the modern one- that of falling in love and other such lovey dovey stuff

But back to your point- about Olympias- I think that Alexander in early years had an unusually close relationship with her- I don't mean incestuous- just very very intimate but here I go hypothesising again. We'll never know the truth. His leaving the kingdom in a kind of self-imposed exile with his mother to Epirus after the Attalus brawl does indicate how close they were thpugh-
An Oedipus complex? Come on, Freud analysed the classic Hamlet in search of traits of an Oedipus complex in Mr Shakespeare and asserted that it was clear- Hamlet's repressed desire and inability to kill Claudius and to bed Gertudes was proof enough for him- how people love to play with psychology!! It is a modern pastime. I think this is even more ridiculous with Alexander.
Renault suggests that Philip's desire to take youthful Alexander to the gardens of Midas- Mieza was simply to distance the boy from his mother- it certainly is an interesting point of view and indication of her powerful influence over him.
Best regards,
Dean.