I don't even know if there's a good explanation why she didnt know he was alive.
He has the same face, same name and he even was still a companion of his other childhood friend until recently and yet she didnt fucking know he was alive!?
I was a little confused about Dia's "I'm a boy" since it's been a while.
It seems she presented herself as a boy to Sieg (she is not fooling anyone tho. Well, maybe Sieg? not sure). That's why she is winking Lastiara, to play along with her.
As for what Sieg thinks, it seems he wants to ignore...
"I'm not trying to snipe this series in any way"
Oh, please do it if you want to snipe.
This manga has been cursed from start to finish, with GD scans dropping a chapter every 6 months, Rightdark being a fucking disastrous MTL (he doesn't proofread anything), and UTOON paywalling chapters...
"Does holding hands with another man count as cheating?"
"If MC does the same with Ayaka does it count as cheating too?"
"Is it the feelings of the person performing the action or the person observing the other person performing it that determine whether or not an action qualifies as cheating...