Ok, I get it now. The second girl in the comic is the girl's sister. They both liked the same guy but the second sister caught him. Now she's congratulating her. The onii-chan should be changed to onee-chan
Or she had incestuous feelings for her brother. I don't see why she set herself up for failure. Marriage is difficult to hide and considering she had the type of feelings that went past siblings I can't fathom that she didn't know of her brother's fiance.