It makes sense now about the family business. You can mention it but If you could find a way of phrasing it such that it doesn't seem that your cv & gpa were so bad that you couldn't get a job as an engineer so you had to settle for family business but rather its tradition or it was just your decision to work in the family business.
About your Uncle,that's tricky. Im not sure is it possible to mention him as an inspiration without saying he is your uncle? All in all you need something to show you didn't wake up one morning and just decided you were going to pursue masters in petroleum engineering. If someone else here could give their thoughts on your SOP that would be helpful, sharing ideas and all.
I was recruited by a oil and gas service company and been working as a field engineer for 2 years (graduated in 2012). I want to advance in this industry and i feel a master's in Peng will help me do that.
As for grad school, do you want Meng as title of the degree or a course based degree? I sent emails to universities in Us and canada asking about the program and also funding for international students like me, so those that gave a positive response i am applying to them. I am applying to University of Louisiana at Lafayette, Louisiana state university, West Virginia University, University of Regina(its in canada) and University of Texas at Austin. All of them have course based degrees. You can check their websites and see if they are a fit for you.
Other schools in the Us that you may take a look at:
University of Oklahoma
University of Wyoming
University of Alaska
Colarado school of mines
Texas tech
Stanford