The farming background is relevant. It helps explain how this particular blood family operate. This is not the same as saying all farmers are greedy.
MIL's power seems to be working, but it is your DH who has to 'break the chains'. He has to grow a set of balls and stand up for himself and his family. Courts are frequently littered with farming family disputes and they look on favourably on those who have given up another life to work for 50p an hour on the farm. An internet search will prove this and for you both to have committed so young to this lifestyle will speak volumes to any arbitrator.
If they have added to the farm over the years then a split on death might be viable. If two families can be supported on all the land then a split might be OK, or DH can work in partnership with his sibling.
Your biggest problem is DH not standing up to his MIL. He should do and you should work together to keeping his inheritance. Yes, he does have an inheritance and this is why the farming background is relevant.
If he loves his 'own' family his mind should be thinking 'we will stay together, we will fight for what is ours, we will work with my sibling if we need to, but if we cannot we will go it alone, but I will not allow our hard work to go unrewarded and I will never allow my family to be split up'.
Believe me he will find it easier to split the land in court than you believe.