Clear out something for me please? Regina is the duke's sister? And this old grandpa cryin...

I might be wrong but I think Regina is the Duke's older cousin...? Not really sure but that's what I remembered and yes, that was Regina's father.

Regina and Philleo are cousin

Younger cousin but yeah you're correct

So... the Phileo's aunt by blood is this Grandpa's late wife. His "new wife" is his second wife. Regina is his daughter from his first marriage and ran away with a man, which is not exactly a proper behavior... even more so when she is considered a member of the Boleoti femily. As he said, she could even be considered a traitor for that..
Considering timeline and ages he was probably already remarried whe she ran away, and that probably gave a blow to his family's reputation. That would explain he not searching/looking for her after she ran even if he loved and missed her. His current wife and kids would probably be affected by her running away too. Hence why they wouldn't be too happy if the daughter "born outside" from "the shame of the family" came back.
Think of it this way, Regina's behavior and Leonia's identity as her daughter is something so out of the ordinary/shameful that not only it's been kept a secret but that Phileo thinks is better for everyone to believe she is his (literal) bastard daughter brought in instead of them knowing the truth.
As such... if she visited gramps as the Duke's daughter and gramps was too affectionate to her in front of his family they might suspect about her identity... and that'd be bad because that makes her a possible heiress to their family since she's the daughter of the eldest daughter and has the blood of Boleoti... pushing her closer to the posicion of sucessor. Even more so since Phileo is single and will marry and have a "legitimate sucessor" one day. So even is he keeps raising her as his daughter she can become a threat one day... even more so when she is receiving heir lessons.
So... even if his new wife and kids are actually good people... her identity is... awkward to say the least and it would benefit no one if it was found out... and as such it would be best if he went to the Duke's house to visit her instead... after all he is Phileo's uncle and technically her granduncle... reason enough to visit them.

Thanks for explaining all that

No probs hun ^^