Ok, people, let's get off the subject of how the OP feels about her brother's girlfriend.
Like many, there seems to be more to the background of this woman than we are privvy too. The OP is not the woman's doctor, she was only stating what she heard from the woman's lips herself or her brother's lips. And it sounds like most of what the woman says is bull, expects everyone to believe her, but we all know it is bull crap. Yep, my husband's brother's girlfriend is exactly like that. He falls for everything she says. We all know it is bunk, but we say nothing. She is his choice and he has to live with her. We are polite whenever around her, but seriously, if you all sat with us one day at the kitchen table at one of our family gatherings, you all would want to Female Dog slap her too!
Ok, as for the OP's question, you offered a baby shower and you should go through with what you offered to do.
It does not matter if this is baby number one or two. It does not matter that she is a total brainfart.
What matters is this is your future brother's child and will be your niece or nephew.
"Well, I am now. I generally do dislike people who manipulate other people into becoming a parent."
Regarding this statement from you OP, I understand where you are coming from and what you mean, but honestly, no one forced your brother into dropping his pants and doing the deed. When doing so, he knew that becoming a parent could happen.