Lately I've been having the urge to tell my cousin she's unreasonable regarding the presence of her baby on social media.
What is she doing that makes you feel that she's being unreasonable?
She is a simply being a idiot online.
I have family that makes annoying Facebook posts too so I get it. You should not ask someone to stop doing something that you will not stop doing yourself. Doing so could create tension and make you be seen as a hypocrite.
They should just stop doing things that offend me.
I understand you are a bit annoyed with what she's posting on social media. But there are ways you can unfollow her so you don't have the view that information that she's posting anymore. That way you don't have to confront her and no issues will arise.