Originally Posted by Lon
I agree with A Bit Much... It's not your role to discipline or raise her teenage son, he is nearly the age of consent and so whatever impression you have on him won't be a a parent/guardian or any kind of authority. Having you in their home takes away your role as man of your house, that has been relegated to this boy. If I were you, and I were otherwise happy with the relationship, I would maintain separate residences until her son becomes a man and moves out.
It's not selfish at all, as others have stated. You are just at a place in your life where you know what you want, and don't want. Never having kids of your own, and then walking into a situation with a 15 year old, is like walking into a war zone. My advice is, don't do it. Be smart about moving forward with this person, if you choose to do so. As mentioned, wait until the kid is out of the house and then blend your life with hers. Good luck!