My parents got divorced when I was 7 and my mother and I moved in with my grandparents...so my situation is a bit different, but I'll spit it out anyway.
My dad was around, but not. He would call me about once a month (I rarely called him...I don't really know why) and I would only see him once or twice a year even though he lived 15-20 minutes away from us.
When I was 14 my mother remarried to a police officer. He was an odd man who took the step-father stance of "Your kid, not mine" and even though he taught me how to drive (he gave me a LEO defensive driver course) he didn't really do much else.
I basically taught and figured out for myself how to do "man things" like shave and stuff like that.
I was never close to him, and now that he's a former step-father I can say that he is a prick and a sack of shit.
When I was about 20 I would say is when I finally started to form an adult relationship with my dad. I think the biggest problem for us was that he didn't really know what to do with kids, but once I became an adult we had common interests and it was just easier for him to relate to me.
So with that tiny ass back story, the advice I'm going to give you comes from the experience of what to NOT do.
When they're at home with you, you're the man of the house. This does not mean they have to call you dad...but they need to learn how to respect you for that. You're not trying to be their dad, you're trying to be a strong male role model for them.
Do everything you would do if they were your kids, but expect the standard "you're not my dad" shit when they get upset with you. Shrug it off. You're not a fair-weather dad like their biological dad. You're the motherfuckin' rock.
Be there for your wife. Be there for her kids. Don't let her raise those boys by herself. Pull your weight, do your part.