snowkitten I moved about 350 miles from one end of the country to another when I was 12.5.
I was aware from a young age that with my DFs job moving might be a possibility.
I never wanted to move - I used to talk about not wanting to move.
I don't think it can be your DDs choice - therefore I would not recommend that you ask her - tbh what 12 year old who is happy in their school would say - yes great idea let's move.
You have very very good reasons for moving - the family support will be invaluable to you.
I think that you need to take the decision to move and then once you have you take stress and uncertainty away from your DD.
It will be so much easier with mobiles/internet for your DD to keep in touch with her friends. My best friend and I started off writing weekly - not too many phone calls as these were deemed too expensive!
I think you should look at planning how your DD can keep in touch - negotiate internet time, texting allowance - agree to have friends to stay once every 2 months or something?
Your DD will make new friends and keep some of the old ones - she will be at an advantage in the future when it comes to college/uni or starting a first job.
I survived my move and have wonderful memories of both places I lived in as a child.
Condolences to you and your family BTW.