So in their novel, it was hinted he knows, but it wasn't confirmed (imo he definitely knew by the way he was looking at the)
From the novel ->
The two stood waving goodbye until the carriage became a small dot, then siqhed.
"..It seems our fathers don't know?"
Cecil looked up at Cyrus with anxious eyes.
Cyrus looked down at his lover and stroked his forehead Though he couldn't precisely understand what those green eyes meant, he felt obligated to reassure his lover.
"They don't seem to know."
"Thank goodness."
He lurks in places lol
WHAT THEY END UP TGT NOOOO THEY ARE BROTHER WHAT -
Does edwin know from the start about Cecil and Cyrus?