Try to look at it more like you have two older children and a baby to take care of. If you had two older children, they would need ferried about to parties, activities etc. This is the life you chose. You chose to be stepmum to two children and have a baby. Your life right now is partnering with your DP every second weekend to take care of his two kids and your baby together. When a baby is young, it's hard work trailing them about to accommodate the older kids but that is part of life. Childrens lives don't stop because a baby sibling has come along.
RE his ex being a nightmare, block her and only communicate via e-mail. You don't need to communicate with her but you can say if she absolutely must contact you, she has your e-mail.
As soon as you get together with someone that has kids then make a series of decisions to commit to that person, you are also taking on their kids so as I said above, you need to look at it like you are a parent to these kids those two weekends a month as if they are your children and ferrying them about is par for the course. I don't think saying you won't do drop offs and pick ups at all anymore will help as you'll just exclude yourself but in a conventional family when a new baby comes, sometimes the mum says she is too tired to ferry the kids about that day and the dad does it. Look at it like that. Say to your DP that this day you are tired and don't want to drag DD about and ask him to get the bus to get his kids or ask his mum or dad to take him.
The quicker you see yourself as their parent on the weekends they are there and adjust the situation to make it so that you are a family but with a young baby that you sometimes need to just give attention to while everyone else goes out, it will get better.
Even women that have all bio kids and a young baby sometimes just needs to say to the kids ok, your dad will deal with entertaining today as I'm tired or the baby is unwell or it's not a day to be dragging your little baby sister about but tomorrow we will do something fun. Older siblings don't mind that a baby sometimes needs a little bit more attention at times but as long as they aren't near being excluded, they will be ok.
It will sound so yuck to say this but I'll say this, you need to put your stepmum hat on 4 days and nights a month. You need to get yourself into the mindset that you are a parent to 3 kids 4 days and nights a month, not just to one. You will also be a parent to 3 kids and not one at some Christmas's and other occasions as well. I don't think it should all be on your DP 'because they're his kids'. I think that's an awful attitude to have. You need to think what it would be like if you split from your DP and met someone else and they treated your DD differently to any kids you had together. They felt a pick up and drop off 4 days a month was too much for them, they hated seeing you cuddle your DD. It would feel rubbish. You would fully expect a new partner, especially one you had kids with, to include your previous child/ren. You need to do that. It doesn't matter how much you play with them or show them love, you resent them and that needs to change mighty quickly or you're heading for ruining your own mental health as well as those kids one's. They aren't stupid, they know when someone isn't enjoying what they are doing