I haven't read your other thread yet but I can see how adding a new small person to a house already full of small people is going to be a major stress on your life. It also sounds like the news has come as a bit of a shock. While you're still in shock you're not going to have the capacity to be happy for her.
Even with her being pregnant there really isn't any reason to put a stop to forming a deeper relationship with her. It might not be on the same trajectory as you were expecting but it doesn't mean that the relationship will be any less meaningful.
As for you never having another child of your own you don't know that for certain right now. There might be someone who comes into your life at a later date with whom you will decide to have a child.
Everything will be ok in the end. If it's not ok it's not the end.