This is more an idea for a scene in a longer story.
A few days before her wedding, a bride teases her brother about when he plans to ask his girlfriend to marry him. He admits he planned to do that soon after the wedding.
The bride then says she would like to be present when he proposed. After some talk she suggests that he propose to his girlfriend when she does the bouquet toss.
On the day of the wedding, the ceremony goes well, although when they are preparing the bride asks the maid of honor when will be her turn. The maid of honor admits she hasn't been asked yet and was getting close to asking her boyfriend (the bride's brother)to marry her.
After the ceremony at the reception, the bride gathers the single young women there and acts like she will be tossing the bouquet. After pretending to get ready to toss the bouquet, she instead turns around and walks to her maid of honor and hands her the bouquet telling her to turn around.
The bride's brother is there on a knee preparing to ask the question.