r/AmItheAsshole: AITA for not letting my miracle baby niece be my flower girl at my wedding?

AITA for not letting my miracle baby niece be my flower girl at my wedding?My (27F) older brother and SIL (both mid 30's) just welcomed their first child a year and a half ago, after YEARS of trying. After many failed attempts, SIL was told that she wouldn't be able to conceive due to a medical condition she has, they finally got pregnant. Since having my niece, the baby has been the center of attention at EVERY family even we've had since she was born. Birthday's, wedding's, family get togethers, you name it. Now don't get me wrong I LOVE my niece, but it can get to be a little too much when my SIL goes on and on about how long they tried to conceive, complications they've had, miscarriages they've had etc. Like a little TOO much info. Many family members have commented on how it's a little bit excessive, but no one has said anything because they don't want to sound like an AH. Anyway I'm getting married in the spring and my brother and SIL approached me last weekend about having my niece be the flower my fiancé (35M) has two children (10M and 6F) from his previous marriage. His son is one of his groomsmen while his daughter had asked to be our flower girl when we told them the news that we were getting married a year ago, as it's something she always wanted to do, so of course we said yes. So I explained this to my SIL when she asked me about my niece. She asked if my step daughter can just carry my niece with her? I said I don't think she'd be comfortable with that considering she's then asked why I can't give that role to my niece, and allow herself to carry my niece down as the flower girl? I said no because I already promised my step then starting going off about how my lack of effort to incorporate my niece is disgusting to her. I should "honor her" in some way since I know how long and hard they tried for my niece. Now I may sound like an AH for this but I kind of got fed up and snapped and said "Incorporate my niece how? By the time the wedding comes around she'll be 2 years old. The ENTIRE family already knows your story about how long and hard you guys tried for her. What more do you expect me to do to honor her?" She started crying and said that clearly I don't love my one and only niece and I'm "letting her down". I said of course I love my niece, and obviously she's going to be involved in pictures and stuff. But I'm not going to let my step daughter down by giving my niece a role she's too young to remember anyway. Well now SIL and my brother are **** off with me for not letting my niece be flower girl, and are running around telling the rest of the family I don't love my niece. My mom had been trying to stay neutral but thinks my step daughter would understand if I explained to her I need to give that role to my firm in my decision though, and my fiancé is thankful that I didn't let his daughter down. AITA for not allowing my niece to be the flower girl?