I think this confirm that Sakine didn't know that Mafuyu was the one who sent the love letter.
There's no reason for her to send her crush to another lesbian.
But man, I wish Chiyoko stop being a people pleaser. She should have told both of them the truth about the letter, especially now that she knows both of them are gay.
Of course someone will get hurt when she doesn't get chosen. But it's better than how she kept lying, trying to be a wingwoman for both.