The thing which strikes me from your post is that This move is centred around what is going to be best for your DH and your DC but what about you?
Other than a bigger house, am I guessing you’re going to be the one who works part time to look after the kids? And your DH will have his family and his friends while you will have to start over again.
And starting out with a new group of mummy friends is vastly different when you move to a new town. Those groups will already be established, a lot of those parents will have put their kids through preschool together, even have attended the same antenatal and post-natal groups, and you’ll be coming in as an outsider, you won’t be an established member of that group, and you may not be accepted.
I’m not going to say anything about your parents and your sister other than the fact that as your parents only have one child who is capable of living independently, it must be immensely difficult for them to see that child move so far away from them, but that’s just an opinion on why they are likely so upset.
But I do think you need to think very hard about whether you’re moving for all of you or whether you’re doing it for your DH and the kids. Because what you feel is also important. You’ll be starting over. Nothing will change for him apart from his address.
And I was struck by the fact you said you will have to choose between your family and your marriage. That sounds like there are potentially issues there if you don’t want to do as your DH asks.
FWIW I’ve been there. I moved because my DH pointed out that it would be better for him (he was commuting and didn’t want to any more,) and better for the DC as there were better schools. I was a SAHM so I moved away from a town where I was known, where my family lived, where I had connections at the school and so on, to somewhere where I knew nobody and the school mum cliques were already established. And for my DH the only thing which changed was his address, and the fact he didn’t have such a long commute to work.
We’re divorced now, and I’m stuck in that town because moving back would have been selfish towards my DC and the relationship with their dad.
If things don’t work out with DH for any reason, do you see yourself living there permanently? Four hours away from all your family and friends? Friends which likely won’t be there any more within six months purely because friendships are fluid that way....
Do think further than the financial implications and the benefits to your DH, and the pressure which your parents are putting on you. This is about you as well....