I don't care what 'good' reason the father had, he is an incompetent failure.
Hiding her from the Royals? Stopping her from becoming a villainess? Preventing a foretold world ending catastrophe?
He went about it all wrong and nothing excuses how she was treated.
Hidden away in the estate with no outside contact, fine; But why shunned by the family? Why treated worse then the cat?
She could have been having family meals, played with brother, had tea and did crafts with mom, and spent time reading with dad. All of those are simple and let her know her family loves her and she is being protected.
Her room didn't need to be barren either, toys and things have a simple workaround; make semi regular donations to orphanages and schools, insist the goods be delivered to your estate so you can deliver them personally, she gets gifts, the children get stuff, and the family's public image goes up.