There's a reason you didn't have contact with your mother for 15 years.
And that started before any these children were born, by the sounds of it, let alone the ones that are biologically yours.
Your dislike of your mother seeps through your posts. That's not a criticism, I'm not overly keen on mine 
But that dislike makes me think - why are you even bothering? You don't like her, attitude to the older children or not. Go back to your reasons for being NC in the first place - they're probably still valid.
On her attitude to the kids - it is unacceptable to treat them differently outwardly.
However, in sympathetic to her not feeling the same way about them. Many many relationships break down. Now you're going to tell us that even if yours did, they're your kids so you would stay in their life... but you can't guarantee that. I don't even think it's a case of getting married - marriage doesn't guarantee no split. If I were her, I would be aware that I could lose these grandchildren, and in fact statistically had a high chance of doing so.
My XH's stepfather has been a proper grandfather to our child. But when his mum split with him, bang. My XH and my XMIL do not consider him to be related to our child at all. He only has contact now because I arrange it. Because I think that his actions for 12 years meant more than biology. But it doesn't suit them. XMIL because she no longer likes her XH (fair do, she has reason) and my XH? Well, he's a selfish arse who can't be bothered. I could very easily have left them to it (honestly, I don't like XFIL either! But he's a good grandfather). I have every sympathy if your mother doesn't see these children as related to her, or as guaranteed permanent features.
I think your attitude that you think it's disgusting that she sees them differently is completely unreasonable.
But I agree that she should make the same effort for them all.
Should you go NC because she won't? I say, don't complicate your thinking! Go NC because there will have been a good reason for it before.