Sister-in-law as Flower girl?

I am getting married in 9 months and I have already pick out and asked my bridesmaids and all of them have said yes. My fiancé and I have lots of friends but we didn’t want huge bridal parties. There is only 6 on each side, both consisting of mostly family members. I did not ask my sister-in-law for many reasons, one of the main ones being that I already had another job planned for her. I want her to be a flower girl. She recently married my brother and I was part of their wedding as a bridesmaid. I had not asked her to be a flower girl yet because their “honeymoon” phase was not over before I left for school. Recently she had called my mother asking why she had not yet been asked to be bridesmaid in my wedding and felt very shut out from my planning. At this point in time I had only been planning for 3 weeks. She has never been supportive of my engagement and she was actually upset and cried when I got engaged because it was “taking away” from her engagement. I waited 7 months to begin planning my wedding because I was waiting until her wedding was over so I would not step on her toes. I am trying to enjoy my time planning my wedding but she makes it feel impossible when she tries to make everything about her. That being said, should I make her a bridesmaid, flower girl, or nothing at all?