Monica, I'm confused. I went through the test 3 times, once for England and Wales, once for Scotland and once for Northern Ireland (I don't know where the OP lives, of course, but those are the only places provided on the site) and in no case did I see where the half-sister would "get everything."
Oh wait, I just saw your other post, where you said you were assuming this would be after the parents were gone. But it might not be, as much as I know the OP might not want to hear that. As painful as it may be, Elizabeth, I think you have to consider all scenarios - if DS (dear son) and DIL, sadly, predecease you and DH, if you and DH (sigh) predecease them and even the half-sister predeceases them, as well.
And you need to check the laws where you live, I believe. While I trust that Ana and Iaincam are correct (and in fact, this is how it is where I live), the test shows that there are some differences from one country to another.
But, hopefully, in time, DS will, in fact, make out wills, after all. Or DIL, who seems interested, will at least do her will, either while he's still living or after.
Roses. if DS dies interstate, I don't think the fact that he is estranged from his half-sister will matter to the courts (I may be wrong though). If he makes out a will, he can, if he wishes, of course, make a point of saying she's not to inherit from him or just leave her out of it. But if he doesn't - well, that's the problem then, isn't it? He forfeits control over where his assets go.