I spent less than a paragraph talking about my socioeconomic status, but my PS was peppered with references to it throughout. In talking about other relevant subjects, I would hint at it (e.g. I made a joke about living with my mom, brother, and a legion of cockroaches who thought they owned the place, while discussing how my mom wanted me to be a doctor). I talked about my Mom's aspirations for me, and how trying to follow that wasn't good for me. I moved on to talk about how deciding to pursue my own interests led me to achieve a great deal, gain experience elsewhere, and eventually a decision to attend law school.
Don't beat them over the head with some sob story about growin up poor; they've heard it before and you might sound like you're trying to buy sympathy. Don't make it the primary subject of your PS. Talk about other significant achievements, and how your SE background influenced your mindset and actions in pursuit of those achievements. Remember, you're tough. You went through a lot; you enjoyed the good, overcame the bad, and you're not sorry you went through it all because it made you the person you are today.