We have not any confirmation that Stacy is MC's half sister ,we only know that they played together and they were best friends as kids but that is all,if She was Margaret would have told the MC that night they met through a video call

You're ignoring BD's (not so) subtle winks at the incest enjoyers. He loves handing out that hit of copium.
Margaret said the dad and Jessica (Stacy's mother) had been banging after MC was born. That for years after he was born, the dad had semi sorta kinda maybe remained faithful because he finally had a son. She also slipped in the one line that said they had been fucking around prior to the MC's birth.
No indication of how long or how many times or even what the gap was between their fling and the MC's mom becoming pregnant with him. Add in the is she/isn't she the sister for so long and how they didn't seal the deal after they learned of Cedar.
Is Stacy a sister? Highly unlikely because BD likes to string fuckers along. Just as he still adds in kissing with Azel. Cedar constantly laid out in "FUCK ME NOW" poses. Elizabeth eluding to but not saying outright that Willow wants to fuck MC. It's all part of his con game.
Temp gets too hot from his antics and he throws down a morsel to one branch of his cult or another. This time he used the mental illness guilt trip card. (Anyone shocked that a Dev renowned for his stories of suicide and survivors pops in here to defend BD just prior to Daliah's story dropping?)