Well, it would make holidays very interesting...
If you were to go into a full relationship, and eventually get married, your ex-step-brother would be your husband, your ex-stepmother would now be your mother-in-law.
For your boyfriend, his ex-step-sister would be his wife, his ex-stepfather would now be his father-in-law.
Your half-sisters would be your half-sisters, plus your half-sisters-in-law. For both of you!
With ALL of the people in Colorado, you are attracted to your half-sisters' half brother. Are you sure that you are in love with him? You are so newly out of a bad relationship, this seems to me to be a sort of rebound relationship. I'm not saying that you aren't really in love with him, I just think that you should put this relationship on the back burner for a bit. Be on your own for a bit. Then try dating some other people, who aren't past step relatives, and see how you feel in a few months. If you are really right for each other, it will work itself out.
Hugs
M.