I DO think you have to look ahead. And here's why.
We moved from Illinois to Missouri when my daughter entered 6th grade (well, to be honest shortly after the year started). In Illinois she was a "young" student - typically the youngest in her class as she has an August birthday. So there were ALREADY kids that were a full year older than her.
In Missouri, she would have missed the cutoff. So, when they looked at her birthday the counselor said - "you mean 5th grade- not 6th". I said "nope - we're from IL she is currently in 6th grade". She sighed.
So - now she's in 7th grade and still 12. There are kids IN HER CLASS that were "held back a year" in Kindergarten THAT HAVE JUST TURNED 14!!!!!!!!! There are 8th grade boys that have BEEN 14 for months. That are considered potential "boyfriends" for my 12 year old. That are in her social circle.
So - if you start your kid early-ish.... well, you're looking to have conversations with her that are going to be on average 12-18 months AHEAD of her developmentally. She will watch PG-13 movies, potentially, when she's 11. Because ALL her friends are. OK, OK - just put your foot down. Tell her she can't be friends with "those" kids. I hope you're one of those moms who feels like a social life isn't important then (and there are many, so you may be one of them) because if you treat her like an 8 year old around her 10 year old friends, well then she's the baby. If you treat her like she's an 11 year old around her 13 year old friends she will lose her friends.
You may not think this is a big deal. And maybe it's not. I look at how important the friendships are and how important it is for these kids to be "the same"..... it's very important.
So - either way is fine. But don't send her "early" and then expect that there isn't a price to pay. She "loses" that year of childhood. She BECOMES a kindergarterner. and the difference between pre-K and K is NOT as big as the difference between 11 and 13 - when dating and hormones and movies and TV is involved.
If she's bored academically, then find ways to challenge her. Get her in the RIGHT academic setting. But do with kids that are her same developmental age. Which is puberty, boys, dating, friendships, sleepovers, driving etc. THAT'S the part that matters for her to be the "right" age. Don't make her have to deal with all that before she (or you) are ready.
I will also say - she doesn't get to go to kindergarten just because her brother went and so now she wants to go to. She can't drive when she's 15 just because her brother is driving. you don't drive until you're 16.... even if she COULD.... is that what's best for her????? No. Similarly, I wouldn't let her start kindergarten just because she wants to do what brother does. They are 2 different people. That's how life works. If you put her in situations before she's ready because she "wants" to be like brother THAT's what's not fair to her. Let her be a kid.
Just my $0.02.