Seems like some commenters are confusing "Won't you become my daughter?" for something like creepy pedo shit.
The reason why the prince wants to take in Ellen is for the royal family now and the future:
1. Vankreift family holds renown and influence as being the family of the hero, and tying them up with the royal family through marriage will secure the royal family's political position. I think that if left alone, there's a risk that some upstart (could be anyone) a few generations later will use the Vankreift renown to start a coup.
2. The royal family's right to rule is based on the spirits' favor, which they already lost generations ago. If Ellen were to marry into the royal family, the royal family down in the future might just be favored by the spirits again.