i should be allowed to post my friends baby sister on 4chan

This is essentially defamation. A baby lives a harmless life, and then posting pictures to a NSFW section will essentially harm that baby’s reputation or create the false representation that a baby already wants be part of the NSFW community (clearly illegal!) 

You are either 1) asking others to make NSFW of a baby or 2) promoting your “baby” in an NSFW section. Normally, sites must verify that people in NSFW sections are 18+. Clearly a baby is not.

I don’t really need to explain this, because this is essentially unethical.

But the pic I would post would be as normal as it gets, so it would have zero effect on the reputation of the small child, who doesn't even have much of a reputation to begin with.

Also, posting a baby’s picture puts the baby at risk for digital kidnapping (baby roleplaying). Someone can easily generate false details about the baby, claiming that it it his/hers when it actually belongs to your friend’s parents. This basically loses control over their identity [2]

Also, from that same source:

“one site offered at least 45 mil­lion images, half of which were pho­tos of chil­dren tak­en from social media accounts. The pho­tos were of every­day fam­i­ly activ­i­ties, but were accom­pa­nied by inap­pro­pri­ate comments”[2]

Surely that is a risk of posting your friend’s baby sister to 4chan. 4chan can easily attract inappropriate comments to children, especially on the NSFW /b/ section. Clear defamation here, three risks included.

No, I'm putting it out there to piss him off and make him THINK it will get more out of hand than it actually will because his dumb, ignorant ass doesn't understand that not everything on the internet is what it seems at face value.

You can do mildly unethical things and still make it to heaven in the end. This point is useless.