You are no different to most mums in this world, you want the best for your the child you love so intensively. That's normal. However, this love seems to have tinted your perception of reality.
It's not true that parents take things away from their elder kids just so that their younger can have more. It doesn't work like that. I don't know one child who was told that they would have to stop and activity they were committed to, just so that one of their parent could spend more time with their new sibling, especially when that parent spend time with that sibling already that they don't get. It just doesn't happen like that because you wouldn't shift the happiness and security of one child for another.
What gives up is quality time between the couple. Most parents I know spend their evenings and week-ends juggling, between work, kids' activities, housework, and allowing each other a few breaks. In your case, it is to sleep later in the mornings, for your OH, it's his time at the pub. What becomes an absolute luxury is quality time together as a couple (and why many end up breaking up sadly).
It is without a doubt the consequences of kids doing more outside of the home, as well as couples not willing to accept the reality of being parents and that the time will come when they will have more quality time together (and wish they had less :)).
You are extremely defensive and come across as justify everything to suit your needs. You are clearly unhappy so there's nothing wrong in talking to your OH and see what he can do different to help a bit, and giving up some time at the pub is reasonable, but similarly, you need to accept that you can't have the perfect life you imagine and that his kids do deserve what their parents committed to them.
It's great that your SC are active outside of the home rather than glue on the Xbox or phone. It's great that their dad is encouraging them to do so. One day, it will be your child who will want to do these activities. In all your heart, can you say that you'd feel to guilt if you were to tell her she had to stop and activity she loved and that you take her too and enjoyed going to see, if your OH suddenly said that he wanted to spend time with you during these times?
As said, I don't know one parent who has told their eldest that they had to stop and activity so that one of their parent could have more time with their younger ones. They share their time, which is what your OH seems to be doing.