I was always the honor student and stellar athlete. At least most of the time.  My brother worked very hard and eventually surpassed my athletic performance but never even came close to my academic performance.   I am six years the elder.

I started my own company at 21.  I sold my share of the company to my partners when I was 26 in order to finance engineering school and the completion of my undergrad degree.

My brother ended up following me into that program and we graduated together with our BSEE.   A couple of years later I decided to start an MBA.  He came with me on that adventure as well.  We graduated together with our MBAs as well.  It was a quest to keep him in school for both undergrad and Grad school.  He struggled.  School has never been difficult for me.

Flash forward 26  years, I have had a successful Director level career.  Little bro has been at the executive level for 10+ years and has been approached by a $10Billion company to be their COO.

He is far more successful than I am professionally.  I am very proud of him.   I surmise that his success is a factor of how hard he has had to work for his success.  As successful as I am,  it has always been instinctual and relatively easy for me.  While I work hard, I do not deliver the unparalleled focus that my brother delivers.  So , I lead $500Mil - $1Bil organizations and he leads multi $Billion corporations.

Our parents expected us to do our best and never compared us.   A kid that struggles will learn things and lessons that a kid for who things come easily will never learn.

My younger brother certainly has compared to his big brother.

My youngest brother passed away when he was a toddler.  There is no way to forecast what his life would have been had he survived other than to know that he would have been raised with love by our parents and had two older brothers who would have his back.  One older brother who does pretty well and the other who is a .05%er.

Just tell her that there will always be people who are better than her at certain things just as there will always be people who are not as good as her. It is just life and something to get used to. It shouldn't be used as an excuse to not try, but a reason to remind ourselves that where we are is ok. Remind her that just because she is not doing the same thing as her siblings does not mean that she is not smart and that she is smarter than most of the poplulation with all of her A's.

Tell her that no-one care who did AP classes and who did not beyond college admissions people. Later in life what people care about most is how you interact with others and that she by far is the smartest communicator of all your kids. No-one is asked about AP classes at interviews especially once they have a degree, what they are looking for is can they do the job (which 80% of interviewees can) and will they work well with their co-workers and that is where she will truely shine. Being a person others want to work with is the best skill to have and will really differentiate her. 

Her social skills will be so valuble to her in life. She will have a life filled with cowrkers, friends and other people who like and care about her and all of those people are potential referees for job positions and oportunities for networking to find great jobs. Not only that but she will have great people around her and be able to fit in almost any environment and thrive. 

Exams and grades seem to be the most important thing in the world when we are in the middle of them. And as parents it is easy to reinforce that impression of the world to our kids because we want what is best for them. But if we can stop and look at those people we know of personally and famous people in general those that are living lives that are personally fulfilling are not always those who got the most advanced and best grades in everything. I know for my part, I got great grades and a couple of degrees that I could throw on a resume but my life is not nearly as successful or personlly fulfilling as many people I know who struggled with education or could not afford it and found their own way to make a great life for themselves. 

Ultimately she has her own journey and path to go down and it may end up looking very different to her siblings or it may be similar but either way is ok and you know that she will be happy as long as she does what she is enthusiatic about for herself and not try to compromise and replicate what someone else does because she feels that is the only version of successful. 

I feel like a failure compared to my siblings who are living great lives with amazing jobs and a couple of kids each. My parents certainly feel that way I am sure, they even prefer my siblings' spouses more than mine. (I am also the third and youngest child).

I hope that this does not apply to your family OP, but I will write my experinces just in case it could help. 

It is tempting to compare myself to my siblings and I thought for a long time that I would have a life that looked very much like my older sister's. I always felt inferior to her intellectually and inferior to my brother socially and interior to both of them in the sports world. I always struggeld to do as well as I did. Things that I thought should be easy were hard. But I got there, I achieved the same degress as them (perhaps from different institutions, but the same none the less).

But my life has taken me in a different direction. In some ways I am glad as it allows me to forge my own path and be who I am rather than emulate them. Things might not be exactly where I would like them, but I have had oportunities that I would not give up for a different life. There is also still lots of time in my life for me to make positive changes, which I am doing even if progress is slow. I know that to most people my life looks wonderful on many levels and I have to remind myself of how fortunate I am to stop me from dwelling on the negatives too much. 

What I crave most though is acceptance from my parents.

What I really want is a relationship with them based on who I am not on what they want me to be. I want to be free to share with them who I am and what is going on in my life but I have learnt over time that doing that just leads to judgements and critism and "suggestions". So I have stopped telling them things and stoped sharing my life with them. We talk still, less often but still a few times a month. But that talk is always superficial so that I can protect myself and something that I mostly do out of obligation rather than desire to talk to them. 

Ultimately what I am trying to say is that it is normal to compare ourselves to those around us. But that parents have a large power to direct our way of thinking and that accepting your daughter as an individual is the most powerful thing you can do. As she becomes an adult and takes her own path trust in her and the way that you raised her that she will do what is right for her. Show her support and positivity for her life and her choices as much as possible. That way she will learn that she has value beyond what grades she gets and which sibling she takes after. 

The kid is super smart in things that do not get AP level classes and do not get tested on ACT/SAT.

Some kids, fortunately and unfortunately, are gifted but not in the things that are the current focus of the educational system. The fortunate part is that they can carve their own path and there's not a stigma on a number on a test for gauging their skill. The unfortunate part is that people thing that number on a test is a big deal and only the things that get tested count for anything.

P.S. And people look at me like I'm insane when it comes up that I am encouraging him to go into the arts instead of STEM like he believes is the only way you can go these days.