oh blimey, you've taken on alot there with 2 step kids as well as your own.
First, pat yourself on the back for being a kind and nurturing person, NOT a dictator! Not everybody would do what you are doing.
Main things you can do to change;
Treat the kids as equal human beings; not that you have to treat them as adults but you CAN treat them as equal; equal right to be spoken to, listened to, respected, and trusted. Do you know what I mean? Don't constantly think of yourself as 'up there' as the adult. You can achieve huge amounts of harmony and fun in a house if you treat the kids as equal partners and get them along with you on things rather than telling them stuff all the time.
Don't try and control everything like clothes, food; I've heard it termed 'benign neglect' on here, meaning let them make their own choices where possible, even if it means very questionable outfits!
Expect to do things as a team WITH them rather than expecting them to do things alone then thinking of them as 'naughty' if they don't do it. Never underestimate the amount of INPUT they need; though quite often you can set them off with things and they can carry on.
Basically they need you to
be absolutely specific in what you are asking them to do
Give them options and choices that play to their sense of dignity and control
but mainly RELAX, look at all your 'rules' and then HALVE them! Pick your battles, learn to take HUGE deep breaths, go away and decide if it really matters!
And get lots of back up and support from your DP!