Nana is MCs maternal grandma so they got different last names. Ophelia only found out what his last name was when he filled out the work application.Considering Ophelia is supposedly friends with the titled Grandma she would absolutely know her familes names, including MC's dad.
Nana loves her family so I can't see her not talking about her kids and grandkids.
Which would then mean Ophelia made the choice to keep Sophie's parentage secret from her "friend" and deny Nana more family.
Which would, in the eyes of dear sweet Nana, be a betrayal of incredible magnitude.
Which would also mean Ophelia kept Sophie in the dark about her extended family and denied her a proper familial relationship with her best friend who in turn is her cousin.
It would also mean that Ophelia would quickly go from being a potential fuck buddy to being most hated in the game by 2 of the sweetest characters.
Or it could be troped to hell and we gloss over the years of lies and deceit while pretending Ophelia isn't a complete bitch.
Side note to anyone expecting good writing from grandma's house: I thought red herring was a type of fish up until 5 minutes ago