you are not ready for step motherhood OP
That's not at all fair.
OP's DP isn't ready to be a decent parent to those DC and isn't prepared to share his life and family with her, only to take what helps him.
OP I have DSC who are much younger and I adore them, we have them over twice a week, they're a huge part of my life (don't have any of my own yet). DH is 100% supportive, includes me but doesn't expect anything of me, parents admirably and is helping to raise kind, thoughtful, bright, wonderful children who are a credit to both their parents. We all respect each other and they know their two homes have different rules and while I'm not their mother and wouldn't try to be, I do have a role as another parent and they're totally on board with that.
It's. Still. Bloody, Hard. Work. Sometimes.
And that's with everything going as smoothly as it could ever have done.
If the paying for dinners thing annoys you - and that's totally understandable, he's taking the piss - if the lack of boundaries, discipline and respect continue (and they definitely will) you're going to lose your mind, your happiness and joy, your will to live and your commitment to your DP and the relationship you have.
Head over to the Stepparenting board and have a read of the posts where people thought/hoped/prayed their DP would change, married him, bought a house with him, had children with him, and then ended up miserable and trapped or alone because he didn't.
I'm sure you love him, but this doesn't sound like the future you want, and it's okay to be honest about that and move on.