That's what she claims though. What people say publicly and what is in their hearts are often very different...
Also.... What is that old, sexist, patriarchal saying??? "Poison??? Why, that's a woman's weapon."
However, the complete opposite could be true... It could've been the case that Arenicos actually wanted his daughter Ira to succeed him and the grumpy old stumps in the Senate couldn't stomach being ruled by a woman and killed him before he could publicly announce Ira as his heir...
But honestly, if I were a betting man, I would put my money on Rhagaea - something about her makes me think of Livia Drusilla.
Livia was suspected killing Augustus but not proved.... I doubt she'd kill him, he had no more people to look for on a heir, Tiberius was his only choice by then... And some people say that Augustus and Livia agreed on his death, so was a suicide kinda of, if was true.
But I say again:
Arenico's could have told his wife about his plans for Ira and for the Empire...after his return to Lycaron and the matrimonial reunion to make another baby/heir. In bed Rhaegea, as the daughter of a King, probably mentioned the succession... Arenicos told her and she decided that even if he was old ( maybe 50th), he could still produce another heir, could put her aside and Ira would be given to a political marriage. So, she maybe poisoned his wine after their lovemaking and pretended the emperor was sick or sent the guards away from the door after she came to him, so they couldn't listen to them. ANd later in the night, for dinner when she sent them to call the Emperor they found him dead.