>>17793693 (OP)would it not come from historical dynasties?
Historically the oldest children are set to inherit everything but if the mother dies and the king remarries and has children with his second wife, then the step mothers children will not inherit.
The step mother obviously favours her children over the kings original family and will therefore seek to undermine or plot against those heirs and get her children on the throne.
I certain recall examples from the ottoman and chinese harems, where different consorts would play off against each other to ensure their children inherited the most.
One example from English history that comes to mind is AEfthryth, the third wife of Edgar; Edgar eldest son, Edward, inherited the throne but Aelfthryth had him assassinated in order for her son Aethelred to take the throne.