If the SKs are rude or disrespectful to me, I correct them ASAP. If I'm alone, I will correct behaviour. Other than that it is DF's place to handle his kids.

I'd never put my hands on them unless they hit me. They know not to try that,

Sorry, but I think your counselor is full of it, and so are most books on this subject matter. Most counselors have never had to deal with being a step-parent because if they were, they'd be singing a different tune. In my opinion, that theory of only letting the bioparent deal with discipline and punishment only works if the parent actually steps up and does it. Most of us on here are venting and pissed because DH/DW's lack of discipline that created all of these entitled and rude SD's and SS's. Alot of us have built up resentment because of this. I tried just letting my DW deal with things when they come up, but she just overlooked way too many things in my opinion. So now I put my foot down and if my SD's don't like it, TOO DAMN BAD! My sanity is more important than their feelings.

I think it is straightforward if the bio parent is quite hot on matters of discipline. It is where they are unreasonably lax that it must be very difficult for the step to stand by and watch. My DH has always corrected his girls when they have been out of line, but he has permitted a lot of things that I wouldn't have, when I was bringing up my daughters. I used to say things to him like "I wouldn't have let my DDs do that..." and he found that quite difficult and told me not to say it.
I try and keep my trap shut but you can't always, and neither should you.

Yes, much easier if the bio parent is stricter then if the step is.
I think one thing is having the authority to enforce the rules that both adults in the house agree on, another is discipline, especially if the latter entails any kind of physical contact. When a parent spanks or possibly slaps, there is bond that assures the child this does not mean they are not loved ( in a healthy household). If a step does the same thing, the relationship will most likely suffer. It takes so much to build a good thing with a step, and one tiny thing can make the whole thing take a huge step back.

I absolutely believe that both parents should have the "right" to address disrespect directed at them.
As far as corrections and discipline, that should be left to the bioparent. Your counselor is right; your child didn't pick your husband. She should be respectful of him, but you need to be the parent.

The only way I've seen a stepparent fully and without major retribution do the parenting is when the stepchild is VERY young.

My fiance and I are pretty much on the same page when it comes to discipline with my bio-kids. However, with his little angels its a little different. He usually leaves their discipline totally up to me, which drives me nuts! Daddy is perfect in their eyes and I'm the wicked step-mom! They never mis-behave in front of him, backtalk, name calling (sd8 calls me slut), hitting my biokids. All of their crap happens while dh is at work! Then the hellions deny it when he gets home!
So, he tells me he "can't" do anything about it after the fact. WTF? It made me feel like one of the kids running to him "tattling" "sd8 called me a name daddy" and I told him as much. That's when he told me to take care of it as I saw fit.
Made the times that they are here and he isn't a lot easier on me.

I speak as a bio mom..I think it also comes down on how the child is raised and diciplined before the step comes in. When I married my DH and he moved in our home I told him he needed to adjust to my rules with my 3 bio daughters because this is what they were used to and nothing was going to change. One thing he said to me which really threw me back was that he was here to back me up in whatever I needed with my girls he was not here to take their dad's place, he was here for me and support me as their bio mom. In my home it works out perfect, Im the strict parent and hes the softie by nature so it evens out, but I have had to talk to him about enforcing a more authority with my girls which I dont mind because they do need a male/father figure in their lives. Ive tried my hardest to raise my girls to respect adults and have told him that DH is another authority in their lives and they need to listen and obey to what he says. My girls love him and get along with him great to the point that they listen to him more than me and that makes me feel very blessed to have someone that is willing to share that responsibility because we all know being a single parent is not easy at all.