My daughter was a flower girl at 25 months of age. I went to Michael's and bought a fake bouquet for her and we practiced, practiced, practiced for like 3 months before. we read ots of books about flower girls an dweddings. And she did great at te rehearsal.
But when it got time to walk, no way. She took off into the pews. Her grandmother eventually urged her down the aisle. the other flower girl, age six, utterly refused unless she walked with her dad who was an usher.
So ANY kid can be unpredictable. And at one and a half or almost two, I think your odds of her doing it independently are vvery very low.
What I wish we had done is had her walk with me (I was matron of honor). It would have been really cute! I actually have a picture of us waiting to go down the aisle that I LOVE! I think all the adults got hung up in wha tshe should do and it wasn't really fair to expect her to do it. Is there an adult or older teen she oculd walk with and/or carry her?? I think it is so nice they want to include her!!! But they will need ot make accomodations.
Also, there were other nieces at this wedding. they all got flower girl dresses although they weren't all invited ot be in the wedding. That oculd have issues too in some families, but she can have a very special role, be in the pictures, wear the dress, even have flowers, but not even do THE walk.
As a side note, I have an 18 month old now. She has been walkingofr 7 months and is very bright. ther eis no way she could do this alone.