I can't use the phrase "second-class citizen" either.
This is for a professional blog post, so I'd rather stay away from "red-headed step-child". I can't use "second-class citizen" because I'm talking about a topic with very few similar posts and one related post uses that same term. If I really need to use "second-class citizen" then I'll probably just reference the other post.
EDIT: For those of you who are not familiar with the term, it is meant to be like a child who is cast away and treated differently than the rest of the family. If you watch Game of Thrones, Jon Snow is the red-headed step-child of the Stark family.
I think the term "black sheep" might be similar, but "black sheep" usually means that the person did something to harm his own standing.
EDIT2: I'd rather not get into the details of the blog post because it's pretty niche. Let's just use the following example:
"When you talk about smartphone OS development, iOS and Android are the over-achieving college kids, Windows Phone is the unemployed pot-smoking brother, and Blackberry OS is the red-headed step-child."
I won't actually be mentioning the other competing products, but I just wanted to illustrate what the term would be calling out.