3) Bear in mind that I came here, asking about her idea (this wasn't my idea). Maybe she is only having a joke or floating an off-the-wall idea but she says all we're really talking about are legal arrangements. When I think about it, it doesn't seem all that overwhelmingly weird. We are not blood-related of course. Why couldn't it be the way she wants it if the law does not prevent it... ...and why might it anyway?
Well there lies the problem, you arent just talking about legal arrangements, if it was just you and your gf thats fine w/e floats your boat, you wouldn't be harming anyone else. BUT you're planning on having a child, who in the eyes of the law will be both your child and sibling. You guys never heard of bullying at school? kids being mercissily teased?.. you are or would be setting your kid up for a potential crappy childhood and that my friend should be reason enough to drop the idea and leave it alone.