Toneria believes that Beastmen are superior and omegas only serve as child making wombs for them. As the beastmen have severely low birthrates, he (a relic of old times and values) believes that multiple mates will increase the chances of beastmen babies rather than being with only 1 omega and risking 0 babies.
Toneria is actually their Great Uncle (Grandpa's blood brother) and holds great power in the Siegfried family. Shaia promised to become the next head and token child of the Siegfried's in exchange for Hugo's freedom basically. I reckon the parents are trying to oppose Toneria and for now, Shaia is just playing along (making Toneria think he is on his side).
I know but it’s just not fair!
The hell is he controlling their lives for, and what he should have harem? What’s his parent’s role in this???????????