I would ask MN to move this to the relationships or property board as this isn't really an ethical issue as such, you might get more replies then?
The thing is I don't think either you or your partner are wrong as such. You have some valid points about wanting more space, she has some valid points about proximity to family, finances etc. And I can see that while you feel you've discussed this in the past and now is the right time, she may feel that it's gone from a theoretical discussion about moving at some point in the future to 'I've found the house I want and we're moving there' quite quickly... I don't think she's totally unreasonable in saying that with a 2 and 4 year old you could stay where you are for a good few years yet (people do successfully bring up children right through their teenage years in flats believe it or not, it's quite common on the continent, it seems a peculiarly british thing to think you must have a house and private garden if you have children). But your point about not wanting to disrupt schooling is a good one too (although if the new house is only 20 mins away you could presumably drive her to school from the new house if you moved in a few years) - how do schools local to the new house compare to ones near your current place? If much better where you are now you might be best off staying put, getting her into a local school then moving to somewhere not too far away and dropping her everyday (your younger sibling would then probably also get into the school on sibling priority).
I think basically what I am saying is that you need a really proper adult discussion about the whole thing where you try your hardest to be open minded and listen to her plan and also properly explain your thinking - so e.g. I wouldn't like it if my partner/husband said to me 'I've spoken to a mortgage advisor and we can afford it' - I'd want to go through the exact sums and really understand the 'scheme' and what it means - is it a shared ownership or deposit loan scheme, how much more would you be paying in mortgage a month compared to now, what does that mean in terms of lifestyle and other savings goals etc. Also whether this really is a 'one off' opportunity in terms of the scheme or whether there are likely to be other similar ones operating in a few years time. Go through the schools issue and look at what school the children would attend in either area, how exactly your commutes would be and the journey to see family and any other things you do regularly e.g. childrens clubs or activities. Essentially draw up a proper pros and cons list (which entails both of you admitting that the other's plans have at least some merit) - e.g. Pros of house are more space, owning your own home, Cons are further from family and work, costs more on a monthly basis and so on. If you still really can't agree then would there be some sort of compromise possible e.g. a bigger flat where you live now or similar.
I know that this might be a frustrating exercise - I know how it feels, I have a partner that if it was just up to him would live literally next door to his family (we actually did for some years!) and thinks 20 mins drive away might as well be the other side of the world, I find this so hard to understand as I'm totally different, have never since I left home been closer than an hours drive away and think that's fine, but ultimately I love him and it's both our homes so I would never unilaterally override him, we compromise and both give up our absolute ideal so we are both happy - funnily enough we've landed on living about 10 mins drive away from his mum and dad, aunts and uncles and about 20-30 from siblings (bearing in mind siblings may well move themselves in the future so you can't always totally plan around them). Also, he's slower to make decisions than I am and generally needs what I think is an inordinate amount of time to make his mind up or accept change (albeit he would say I'm rash and impulsive
) so I know I sometimes just have to sow the seeds of an idea and let him cogitate on it for a while, he's also very stubborn so me nagging him constantly or presenting something as a done deal that he doesn't like the sound of just doesn't work. Again, just pretty standard relationship stuff you have to navigate there I think - perhaps your partner is similar, after 10 years you must know how her brain works with this mind of thing? Moving house is a really really big deal for some people more so than others so she may just need more time and to really feel like you are on her side rather than pressing your own agenda here even though you feel it's all for the benefit of the family, if you see what I mean?
Good luck, hope it works out!