Can I just ask, BRDGRL? When you say "introduced house rules and responsibilities" what kinds of things?
Well, he/we did this in two 'phases' - one that was led by DH, in their own home, before we ever lived as a family. Then there was a second 'phase' that happened of course when we all moved into a new home together, and that was more of a joint effort, and that was when I had more of a say in what went on, as it was now 'our' home.
But well before we moved in together, DH started to change things. The kids were given chores for the first time - helping with meals, hoovering, walking the dog. He stopped waiting on them hand and foot - if they were watching tv and shouting at him to fetch them drinks, he started telling them to get it themselves. He cut well back on the amount of 'taxi service' he would provide, and instead the kids learned how to use the bus system. He introduced sit down dinners at a regular time, at a table. (Previously, they did not sit down to dinner at the table, but watched TV with plates on their knees, while DH cooked, cleaned, waited on them, and gave them anything they asked for.)
He also began the long process (with advice from a counsellor) of 'de-spousifying' DSD. That meant both encouraging her to do more with her friends, and having a social life apart from DH, and also taking away some of the adult roles she'd been allowed to play. DSD did not have adult responsibilities - on the contrary, she was waited upon and the expectations regarding her manners and behaviour were appropriate for a much smaller child - but she did 'play' at adult roles. She would not do any cleaning or cooking, but would rearrange furniture, make decorating decisions, and decided what they'd eat and when. She did not 'look after' her brother in any practical sense, but did boss him about and spoke about him as a mother might ("we don't allow Brother to do x", that kind of thing). She told both DH and DSS what to do and got very upset when she was not in control of even small things. She also accompanied DH almost everywhere, including 'adult' occasions and events, at which she would behave as his date (which was painful as - quite naturally - she did not have the social skills and sense of an adult). DSS was frequently excluded or infantalised, as she was part of a unit with DH and DSS was on the outside of that. DH had to turn this all around, and that was (and has been) tough. The amazing part is that is has largely worked.
Since we all have moved in together, there are new rules - mainly about cleanliness and things that affect the baby's safety or well-being (I ask them to keep doors closed, for example). I have been tried to be careful to distinguish between things that directly impact on me and DD, as opposed to things that I might do differently with DD but which are already choices or habits for the DSCs and not really 'my problem'. So for instance, DH did not, and now we continue not to, ask the kids to keep their bedrooms tidy - I do say that they have to keep it down to where they could escape in a fire and that they are not to leave dirty dishes or sweet/crisp wrappers that will attract bugs - but that's a pretty low standard. On the other hand, common areas of the house are to be kept tidy and they have to clear away their things from those rooms every evening - and we are rigourous about that.
I added a couple new rules about things I felt particularly strongly about - no 'hate speech' around the baby (the DSCs are nice kids, but would quote things from southpark or family guy etc), no food in our lovely front room (they have a tv room where they can sit with crisps etc!), no mobile phones at the table...but mainly the house rules are about chores and picking up after themselves.
Sorry for the long reply!