Sorry MCDL, you know that I think you respond with bravery and integrity when you inevitably get strong reactions from posters here, but yet again, I honestly don't think you're truly empathising properly here.
And your way of writing, with all the "...."s in the middle of your sentences and a line that made me howl; "the rock that I perish on..." makes me think you see yourself in the middle of a Greek tragedy, rather than what has been a tragic situation for two children who never asked to be born to an alcoholic and an adulterer.
Of course those kids wouldn't have wanted to leave their mum to go and live with him at that point. They probably felt responsible for her, especially if she went to pieces when her H left. Besides, if your presence was known to them both at that stage, they wouldn't have wanted to live with you too at any point. If your DP was however pretending that he'd left and there was no OW involved, the older one woud have had the savvy to work out that he was being deceived.
Those kids probably had a shit childhood and the money they take from their Dad and you now is no doubt viewed by them as their just desserts after all that. Not particularly edifying, but understandable IMO. But like others have said, they are also behaving like youngsters do, expecting there to be a bottomless pit of money.
I don't think your DP needs to choose at all - and if he does choose you and DD like you think and shuts off financial and emotional support to his older DCs, then he will just fulfil everything they probably believe about him anyway - that he dumped them in an intolerable situation while he chose to make a new family. And you seem to think that "most women" would be happy for a man to disregard his previous family. Really? Not this woman, anyway. Quite the reverse, I'd think he was an absolute jerk and best avoided.
I really think you're going to have to give this time and patience. This is not long at all for people to have forgotten the memories of picking up the pieces of those kids' lives. Yes, they might feel equally badly towards the alcoholic SIL, but they were entitled to think that their brother should have shouldered his responsibility to the children, not left them to nurture two kids whose parents had defected; one through illness and one through selfishness.
It is in all probability why your DP's brothers are a bit more sanguine about things - I'll bet they weren't the ones who had to intervene at the school, mop up all the tears and hurt on a daily basis, like the aunts did.
Just give it time and really try to see it from these people's point of view, instead of calling their behaviour "ridiculous".