I think if the house was entirely his ex-wife's - so either paid for fully by her or inherited through her famiy, then your EX is really only a guardian of that asset for his DS.
If, however, the house was bought by them together and they had an insurance to pay the mortgage off in the event of the death of one of them, then it's not 'hers' it's 'theirs' and as such it's not just for one of his children to inherit.
I do think though, as his DS only has his Father and your DD has both her Mother and her Father - that it is only right his DS should get more of his 'estate' than your DD as she will still inherit from what you are able to provide for her.
I would even argue that if the house had belonged to your Ex before he met his wife, then the two children could inherit equally...
Frankly, though, you don't actually get any say in it. He is entitled to put what he wants in his will and if he dies your DD has the right to contest the will if she thinks it's unfair.
I would set up an insurance for your DD so that when you do die she stands to inherit some money and obviously bring her up to be in a position to not need it if she is an adult.
FWIW - It's not that I think adult children have to inherit money, but I do think when you have children, while they are still children, you need to make sure they would be OK if you were to die tomorrow - of course - once they are grown you should spend it all yourself . I actually think parents should, where at all possible, set up life insurance so the children benefit financially when they die, it's a small outgoing for a large benefit - why wouldn't you?