Cory, Pictish,
I get what you are saying but PP have rightly identified that this is more about her hoping this would all go away. So when he wasnt having anything to do with the child she could pretend it wasnt happening, but now she has to face up to reality and that is what is so gutting for her.
Its like someone pointing out the growing mole on your arm. You know its there but you ignore it, and then one day it becomes so big it can't be ignored and your doctor tells you it is cancer. You knew that all along, but having the facts suddenly staring you in the face is shocking, hurtful, frightening. Thats where the OP is now.
I am not saying its right, just that it is understandable.
A one night stand and she gets pregnant? Either un/lucky or it was a relationship on the burner either before/just after your break.
That is a very good point. Either he should have a DNA test to be sure, or front up to the fact that it wasnt just a ONS if he is so sure it is his child.
I do think that this child is nothing to do with the OP. Her child is its sibling and at somepoint she will have to accept her child and his child having a relationship (as they should), but I dont think that the OP should automatically be expected to embrace this child.