OP I think you are getting a really hard time here.
You need your step back and look at each part of this in isolation there is no standard you just treat them the same in all
situations.
Here is my view, as a step child twice over with my experience of inheritance.
Should you be treating them the same in terms of gifts / outings etc if they are all there at the same time. Yes, probably.
If you are out with your own two DGC and buy them an ice cream or take them to the cinema should you need to do the same to make it fair with the SDC. No, probably not.
Is it ok to pay for expensive clubs for your DGC that live full time with your DD but you are paying for only 2 DC. Yes fine.
If one of the DSC was at the same club and could no longer go because of funding, I’d expect your DD to make a decision on pulling them all out or accepting your help and her making a choice to play favourites with her family.
if you want to pay for your 2 DGC to go on holiday that only they are on (with or without DD/SIL), fill your boots.
If you want to only pay for 2 when all 6 are going on holiday that feels a bit unfair.
If DGC are already in private schooling that your DD can no longer afford and you offered to pay for the two of them to stay there when the DSC are not already in private school, great. Your DD should choose to take you up on that offer or not. You shouldn’t need to have to fund the other 4. They aren’t getting it today so it’s not your responsibility to give it to them tomorrow.
if your DSC are also already in private education and they are the only ones to come out, then it seems unfair to only fund 2. It’s still not your responsibility to fund them all and on your DD and SIL as to whether all their children stay in or not. It’s not your decision to make on allowing only 2 to stay even if you are happy to pay for only them. That is on your DD and SIL. If they don’t like you only paying for 2 then they politely say thank you but no thanks and pull out all their kids.
Should you have to pay for 4 extra SDC to start private school when they are not already in it. Absolutely not. If your SIL didn’t want a disparity between his kids he shouldn’t have allowed his other 2 to even start.
Should you include them in your will. Totally and utterly up to you and no one else’s business. Set this up however you want. Tell them whatever you want. At the end of the day they will only find out what you have done once it’s too late for you to care or see the reaction.
in our case our grandparents did not include any of their DSC in their wills. We did however all end up as one blended family getting on well with each other and did share it out. That was our choice though and had we not wanted to it wouldn’t have happened. We’d have all taken our own shares from our respective grandparents.
They did however treat us all the same in terms of gifts / outings / experiences etc.
Only you know what you want to do and are willing to do. If you are not happy with what they want from you then pull your offer of financial support. Save it separately for your DGC for when they are older and can put a deposit down on a house or something. By that point their dad will have no control over it.
and at the end of the day remember you are not making the decision for a disparity on DC, your DD and your SIL are.