There aren't any set-in-stone traditions about who gets to wear wedding-day flowers. But here's who most couples choose to honor: The parents and stepparents, grandparents, any other immediate family members who are not in the wedding party, ushers, and the ceremony readers. Either way, it's up to you.
I have had brides who gave a wrist corsage to her 4 best friends. I have also done a wedding where the bride gave each of her young nieces a corsage and her young nephews a boutonniere. She was close to them all and she wanted them to feel special at her wedding. It was very sweet. I think each of them grew several inches that day because they felt so grown up with wearing there flowers.
You can give flowers to who ever you want as long as you keep things even. It would not be a good way to start your marriage by giving a corsage to your sister and not your sister in law. That is just a fight waiting to happen. And it is good to remember that you are marrying a whole family. This day sets the tone for so many things that are ahead. Weddings tend to bring out a lot of emotions and when they are all good emotions with thoughts of good wishes and happiness it just makes the day of your wedding even more special. Why would anyone want to take any chances of things not going well over a corsage or boutonniere. Such a small thing in the scope of what is going to happen that day.