I've been with my husband and his son (almost 16) for coming up to 5 years, and honestly, it's only been the last year that we've properly blended.
We have a toddler together, and I have daughters of my own that are teenagers. My DSS lives with us as his mum passed away when he was young.
Before the last year we were very unblended, it was very much me and my girls and him and his son, but it's very different now. Last Christmas my DSS spent the whole day hanging out with us, my family, my daughters, and after we went to bed, all the teens stayed up watching movies together, and that's the first time that's ever happened.
For the last 3 months or so my relationship with my DSS has also come along in leaps and bounds. We have a much more natural relationship now, we can talk about things, I love him and I know he loves me.
I don't know if it was just time for everyone to get used to the situation, or the kids getting older and maturing, but things can change for the better, even if doesn't seem like they ever will.
My husband doesn't treat his son especially different to the other kids though.