I think it depends on the age of the kids primarily. In your situation and with your DSS being so young, you have to discipline because you're the one who's around the most - I would be having a word with your DH though if you think he should be more supportive. As for smacking - I think that's a decision to be made by each one of the parents and step parents jointly if possible so that there is consistency. That's ideal world though and we don't live in an ideal world unfortunately.

My DSD is 15 and has lived with us on and off since she was 13 so it's a completely different kettle of fish. Basically I leave sorting out curfew times, how she's getting home etc. to DP BUT if she wants to go out unannounced I will get her to phone her dad and ask permission/let him know. Likewise, she knows that if her and DP have agreed she is to be home by a certain time and she's not, then I will tell DP. The only situation I would step in is if she was rude to me but touch wood, that's never actually happened so haven't had to cross that bridge.

I have no idea what I would do with the in-betweenie ages if your stepchildren entered your life between 5-12.