Dear H.,
I highly recommend Jane Nelson's book "Positive Discipline for Preschoolers." It will help you learn to understand your kids, and when your children KNOW that you will listen to them and care about their feelings, they'll WANT to work cooperatively with you. They'll TRULY respect you, not just obey and comply out of fear. In the long run, gaining their trust and cooperation is much more effective than all the nagging, threatening, punishments (and rewards!) in the world.
http://www.amazon.com/Positive-Discipline-Preschoolers-Ye...
At this age, they really don't have the capacity to understand abstract concepts like "disrespect for authority", so don't take it personally, or feel threatened when they don't immediately obey your every command. It takes a long long time for them to learn how to express themselves, and even longer if you get on them for "back talk" or "sassing." They just want their voices heard, too, and if their mother won't listen to them, NO ONE else in the world will.
Teach them respect by modeling respect *to* them, teach them to listen by listening *to* them. Validate their feelings and let them know you understand how they feel, even if it isn't an "acceptable" feeling, like not wanting to share. This isn't caving in to whatever they want - it's showing you are listening and understand. Don't allow them to pull you into anger - strive to show kindness and firmness at the same time. (Easier said than done.)
Don't try to solve a problem when they are too upset to comprehend - they can't be rational when they've lost control of their emotions. Get them through the meltdown, and when they are calm, they can be receptive to finding a solution.
If something isn't working, don't keep doing the same thing again and again just for the sake of "consistency."
Consistency is only a virtue if you did something perfectly the first time. It's much better to think about your interactions with your kids, when something screws up, re-hash it in your mind, try to think what you could have done better, resolve to continually improve your parenting. Just admitting your mistakes to your child and apologizing is huge. So many parents are afraid they if they admit they've made a mistake, they look weaker, but if children see you admit a mistake or confess that you could have handled something better or apologize for losing your temper, they begin to recognize it in themselves.
And whenever possible, you should be the one to handle the correction/teaching/discipline, especially if the children are acting in a way that will bring out the anger in an adult. There are times when children will drive you to lose your temper, but you are much less likely to lash out in anger if they are your own flesh and blood, the love is too fierce. Nothing against your boyfriend - it's just a biological truth that you have a stronger bond to your children than he ever can. Most step-parents would never hurt a child, but of the children that *are* abused or... or worse, they are most likely victims of a step-parent. (Not to freak you out - that's just the way it is.)
Good luck!