Himegoto Kakushigoto. Is this really the real title? Have u guys read manga or anime "Kakushigoto". The mc daughter name is actually Hime Gotou. Is it really a coincidence??
The reality is that their father loved both of them equally.
I wouldn't be surprised if Grandpa intervenes to protect his son-in-law's dignity or some other family secret. Like, he wouldn't oppose to the two of them marrying but would still be an opponent when it comes to digging around too much.
Or maybe he knows the truth all along (maybe the parents each had an affair) and he doesn't want yuzu to find out she is also illegitimate.