I agree with redhen. There have to be house rules that you and your DP have agreed on, and there have to be consequences laid out for when they are broken. Once you and your DP have decided what those should be, then they need to be made clear to your DSS. Personally, I think that any adult 'head of household' ought to be able to enforce house rules, remind the kids when they're breaking (or about to break) one, and also to impose consequences. In reality, I do a lot of reminding, but DH usually delivers the consequences. If my DSC were younger, I think it would be easier for me to do both.
'House rules', for me at least, are sometimes different from rules about the kids' development. I won't try to argue for rules about what my DSCs wear, for example - as far as I'm concerned, that doesn't really affect me or my DSD -who is enough younger than the DSCs that it is easy enough to have separate rules about such things for her as she grows up, and if she says later "but DSD is allowed to wear a bikini", I am quite happy to say "yes, but I am not DSD's mum".
On the other hand, I do have a say in rules or decisions about how much is spent on clothes, how they get washed, whether they can be left on the living room floor, etc...If it affects me, my DD, or the family as a whole, then I get to be involved.