It's pretty clear that in your blended family, your usual behavior is "nice," which is another way of saying self-effacing and passive. Your step-children and husband are more dominant, and you feel invisible to them. Typically, a passive person will get sick and tired of being ignored, and then explode in periodic bouts of aggression. This doesn't help. I call it the "exploding doormat" syndrome. It will leave you feeling even more invisible, and make your family even less loving.
Instead, you need to begin developing more self-esteem, so that you can act assertively without being aggressive. A therapist would help enormously, or, if you want a low-cost fix, watch "The Dog Whisperer" on the National Geographic channel. Believe it or not, learning to be a calm assertive leader with dogs will help you behave differently with people. Whatever you do, remember that self-esteem, not control or more "niceness," will allow you to set better boundaries and feel seen, heard, and respected.