Treat others how you want to be treated. Well, and don't be surprised with people want nothing to do with you when you've treated them badly. A lot of people take their family for granted, a circumstantial set of people who are stuck with them no matter what, and who will keep being there for them despite never doing the same in return. Family is more than just circumstance; it's the people who are repeatedly there for you and show up when it counts, who go out of their way for you without even being asked, standing by your side when things are at their hardest. But when the opportunity presents or necessitates, you need to be able to do the same in return.
It's about loyalty, care, and consistency.
By his own account, this "family" showed this now 20-year-old none of the qualities that make up the foundations of what an actual family is. They might be family by legal definition, but they certainly never made any attempt to actually be such. While blending families is always difficult emotionally for all kids involved, it is marginally easier with young children who have a less developed sense of identity. So it should have been possible for his stepmother to accept the young child in her family with open arms. She and her children made no such attempt, treating him always as an outsider and as less than them.
It's no surprise then that now, at 20 years old, he'd rather have nothing to do with them. That ship sailed, repeatedly, over the course of the last 15 years.