12 year old flower girl?

Hi everyone! So I have 8 nieces and nephews and I am trying to incorporate all of them into my wedding. My 2 oldest nieces are bridesmaids (ages 18 and 15). My oldest nephews I am planning to ask to be ushers (ages 23 and 16). Then I was thinking of having the other four serve as 2 flower girls and 2 ring bearers. 3 of them will be 8 or 9 years old but one flower girl would be 12. Is that too old? She is the older sister of the other flower girl so I thought I could tell her that she would be leading her sister...