Well OP, there's nothing to stop you changing your will to favour your eldest and state it's because your youngest will also inherit from his dad (your DP), while your eldest will only inherit from you. You don't need your DP's permission to change your will nor do you need to tell him what your will contains.
Another thought, is it possible that your oldest child will inherit from his father's family? Either directly his dad or grandparents on that side? Even if you don't have anything to do with them, they might still mention their blood relation in the will.
This doesn't mean that she didn't care for DC1, that she didn't enjoy his company. Perhaps look at it this way, your DC1 is a relation of hers via marriage, in the same way you are. If she hadn't decided to skip a generation, would you expect the inheritance to be split evenly between just her sons and daughters, or would you expect it to be split between sons, daughters, son-in-laws and daughters-in-laws? (even if that meant that some couples got more than a single DS/DD). Effectively, your DC1 is a "grandchild-in-law" - a relationship that's via marriage/living together rather than a 'blood' relation. She might have loved and cared for him, and she might well have loved and cared for you, that doesnt mean she should see you as a DIL in the same way as a 'real' DD.
Both your DCs are your DCs, but they are not both your DP's DCs. No matter how well people treat them within your DP's family, this is only his family because you are with DP. If you split up with DP, your DC2 will still be related to these people, but your DC1 won't be in the same way you won't be anymore.