New family/old family.

For those of you who have a SO with a child or children from a previous relationship... How do you make it work? For the past three years I've been dating a man with a 14 year old son who lives with us full time and I often find myself feeling like the outsider invading on their family. His son often points out that I'm not his stepmom even since we're only dating. I always feel like my opinion is unwanted and when I do give it like I'm overstepping. I don't know how to feel or how to act. My boyfriend works a lot and so his son and I spend a lot of our time together and I don't know how to handle it. I tend to refer to "ask your dad" but then his dad always asks my opinion. And if I disagree with something I become this bad person. How do you create a family where one already exists? How do you integrate?