OP - I think YABU to expect a parent to involve their adult DC in the decision as to if they should remarry and have more DCs, it did effect you, but it's not your business. Sorry, but it's not something you should have been contacted about. The jealousy feelings do seem petty in a grown woman, but are understanable, even if they aren't reasonable.
However, YANBU if you feel your dad no longer bothers to think about you and your Dsis since having a new DC, and if he only contacts you when he wants something, that will be hurtful and feel like you're unimportant to him.
I would suggest you move out of his flat and find something of your own, lower your expectations of him, you probably won't get Christmas and birthday gifts or even acknowledgement. Don't confront him about it, you know he'll just go on the defensive and hurt you.
I wouldn't cut him off completely, there's no need to do that, however, you could just make as much effort with him as he's making with you. It might be petty to do that, but I'm of the opinion the person who should make the most effort in a relationship between an parent and an adult child is the parent still.