Finally... Ellena could marry the man of her dreams.
It took her a long time and a detour, but it was well worth it.
Mars's daughter would not be a threat, so Ellena didn't need to get rid of her. Different story if it was a boy. Once Ellena married Mars and gave birth to her own son, she would have to get rid of Mars's son from Emmelyn.
She couldn't let him live because the boy would take away Ellena's biological son's chance to claim the throne.
But a daughter... was not a threat. In fact, she was perfect. As a new father with a small baby, Mars would need a woman to help him raise his daughter, and Ellena would make herself available for the job.
She would get rid of all the other women who might be considered for the role, leaving only her as the most possible candidate.