I too married into a family and my step son was 16 when I first met him. He was pretty not into sharing his father after having him to himself since he was 2y.o.!
Oh well.. that's life. He went thru quite some times and just because I was a bit of a challenging teen I pretty much understood tho it's difficult when one is the grown up and the step parent~ altho he's 30 now and just had his own son 4 months ago so everything turned out A-OK!
Good luck w/all
